Improve documentation of treesit "thing"

* src/treesit.c (syms_of_treesit):
* lisp/treesit.el (treesit-cycle-sexp-type):
(treesit-thing-at, treesit-thing-at-point): Doc fixes.

* doc/lispref/parsing.texi (User-defined Things): Improve
documentation of treesit "thing" and related functions; add
cross-references and indexing.

doc/lispref/parsing.texi

@@ -1619,14 +1619,16 @@ documentation about pattern-matching.  The documentation can be found at
 
 It's often useful to be able to identify and find certain @dfn{things} in
 a buffer, like function and class definitions, statements, code blocks,
-strings, comments, etc.  Emacs allows users to define what kind of
+strings, comments, etc., in terms of node types defined by the
-tree-sitter node corresponds to a ``thing''.  This enables handy
+tree-sitter grammar used in the buffer.  Emacs allows Lisp programs to
-features like jumping to the next function, marking the code block at
+define what kinds of tree-sitter nodes corresponds to each ``thing''.
-point, or transposing two function arguments.
+This enables handy features like jumping to the next function, marking
+the code block at point, transposing two function arguments, etc.
 
 The ``things'' feature in Emacs is independent of the pattern matching
-feature of tree-sitter, and comparatively less powerful, but more
+feature of tree-sitter (@pxref{Pattern Matching}), and comparatively
-suitable for navigation and traversing the parse tree.
+less powerful, but more suitable for navigation and traversing the
+buffer text in terms of the tree-sitter parse tree.
 
 @findex treesit-thing-definition
 @findex treesit-thing-defined-p
@@ -1635,12 +1637,22 @@ predicate of a defined thing with @code{treesit-thing-definition}, and
 test if a thing is defined with @code{treesit-thing-defined-p}.
 
 @defvar treesit-thing-settings
-This is an alist of thing definitions for each language.  The key of
+This is an alist of thing definitions for each language supported by the
-each entry is a language symbol, and the value is a list of thing
+grammar used in a buffer; it should be defined by the buffer's major
-definitions of the form @w{@code{(@var{thing} @var{pred})}}, where
+mode (the default value is @code{nil}).  The key of each entry is a
-@var{thing} is a symbol representing the thing, like @code{defun},
+language symbol (e.g., @code{c} for C, @code{cpp} for C@t{++}, etc.),
-@code{sexp}, or @code{sentence}; and @var{pred} specifies what kind of
+and the value is a list of thing definitions of the form
-tree-sitter node is this @var{thing}.
+@w{@code{(@var{thing} @var{pred})}}, where @var{thing} is a symbol
+representing the thing, and @var{pred} specifies what kinds of
+tree-sitter nodes are considered as this @var{thing}.
+
+@cindex @code{sexp}, treesit-defined thing
+@cindex @code{list}, treesit-defined thing
+The symbol used to define the @var{thing} can be anything meaningful for
+the major mode: @code{defun}, @code{defclass}, @code{sentence},
+@code{comment}, @code{string}, etc.  To support tree-sitter based
+navigation commands (@pxref{List Motion}), the mode should define two
+things: @code{list} and @code{sexp}.
 
 @var{pred} can be a regexp string that matches the type of the node; it
 can be a function that takes a node as the argument and returns a
@@ -1660,13 +1672,16 @@ meaning that not satisfying @var{pred} qualifies the node.
 Finally, @var{pred} can refer to other @var{thing}s defined in this
 list.  For example, @w{@code{(or sexp sentence)}} defines something
 that's either a @code{sexp} thing or a @code{sentence} thing, as defined
-by some other rule in the alist.
+by some other rules in the alist.
 
+@cindex @code{named}, treesit-defined thing
+@cindex @code{anonymous}, treesit-defined thing
 There are two pre-defined predicates: @code{named} and @code{anonymous},
-which qualify, respectively, named and anonymous nodes.  They can be
+which qualify, respectively, named and anonymous nodes of the
-combined with @code{and} to narrow down the match.
+tree-sitter grammar.  They can be combined with @code{and} to narrow
+down the match.
 
-Here's an example @code{treesit-thing-settings} for C and C++:
+Here's an example @code{treesit-thing-settings} for C and C@t{++}:
 
 @example
 @group
@@ -1676,6 +1691,8 @@ Here's an example @code{treesit-thing-settings} for C and C++:
   (comment "comment")
   (string "raw_string_literal")
   (text (or comment string)))
+@end group
+@group
  (cpp
   (defun ("function_definition" . cpp-ts-mode-defun-valid-p))
   (defclass "class_specifier")
@@ -1685,12 +1702,12 @@ Here's an example @code{treesit-thing-settings} for C and C++:
 
 @noindent
 Note that this example is modified for didactic purposes, and isn't
-exactly how C and C@t{++} modes define things.
+exactly how tree-sitter based C and C@t{++} modes define things.
 @end defvar
 
-Emacs builtin functions already make use some thing definitions.
+Emacs builtin functions already make use of some thing definitions.
 Command @code{treesit-forward-sexp} uses the @code{sexp} definition if
-major mode defines it; @code{treesit-forward-list},
+major mode defines it (@pxref{List Motion}); @code{treesit-forward-list},
 @code{treesit-down-list}, @code{treesit-up-list},
 @code{treesit-show-paren-data} use the @code{list} definition (its
 symbol @code{list} has the symbol property @code{treesit-thing-symbol}
@@ -1699,8 +1716,8 @@ to avoid ambiguity with the function that has the same name);
 Defun movement functions like @code{treesit-end-of-defun} uses the
 @code{defun} definition (@code{defun} definition is overridden by
 @var{treesit-defun-type-regexp} for backward compatibility).  Major
-modes can also define @code{comment}, @code{string}, @code{text}
+modes can also define @code{comment}, @code{string}, and @code{text}
-(generally comments and strings).
+things (to match comments and strings).
 
 The rest of this section lists a few functions that take advantage of
 the thing definitions.  Besides the functions below, some other
@@ -1709,10 +1726,10 @@ tree-traversing functions like @code{treesit-search-forward},
 @code{treesit-induce-sparse-tree}, etc.  @xref{Retrieving Nodes}.
 
 @defun treesit-node-match-p node thing &optional ignore-missing
-This function checks whether @var{node} is a @var{thing}.
+This function checks whether @var{node} represents a @var{thing}.
 
-If @var{node} is a @var{thing}, return non-@code{nil}, otherwise return
+If @var{node} represents @var{thing}, return non-@code{nil}, otherwise
-@code{nil}.  For convenience, if @code{node} is @code{nil}, this
+return @code{nil}.  For convenience, if @code{node} is @code{nil}, this
 function just returns @code{nil}.
 
 The @var{thing} can be either a thing symbol like @code{defun}, or
@@ -1727,8 +1744,9 @@ undefined and just returns @code{nil}; but it still signals the error if
 @end defun
 
 @defun treesit-thing-prev position thing
-This function returns the first node before @var{position} that is the
+This function returns the first node before @var{position} in the
-specified @var{thing}.  If no such node exists, it returns @code{nil}.
+current buffer that is the specified @var{thing}.  If no such node
+exists, it returns @code{nil}.
 It's guaranteed that, if a node is returned, the node's end position is
 less or equal to @var{position}.  In other words, this function never
 returns a node that encloses @var{position}.
@@ -1753,8 +1771,9 @@ function doesn't move point.
 
 A positive @var{arg} means moving forward that many instances of
 @var{thing}; negative @var{arg} means moving backward.  If @var{side} is
-@code{beg}, this function stops at the beginning of @var{thing}; if
+@code{beg}, this function returns the position of the beginning of
-@code{end}, stop at the end of @var{thing}.
+@var{thing}; if it's @code{end}, it returns the position at the end of
+@var{thing}.
 
 Like in @code{treesit-thing-prev}, @var{thing} can be a thing symbol
 defined in @code{treesit-thing-settings}, or a predicate.
@@ -1780,8 +1799,8 @@ less or equal to @var{position}, and it's end position is greater or equal to
 @var{position}.
 
 If @var{strict} is non-@code{nil}, this function uses strict comparison,
-i.e., start position must be strictly greater than @var{position}, and end
+i.e., start position must be strictly smaller than @var{position}, and end
-position must be strictly less than @var{position}.
+position must be strictly greater than @var{position}.
 
 @var{thing} can be either a thing symbol defined in
 @code{treesit-thing-settings}, or a predicate.

lisp/treesit.el

@@ -3237,11 +3237,14 @@ The type can be `list' (the default) or `sexp'.
 
 The `list' type uses the `list' thing defined in `treesit-thing-settings'.
 See `treesit-thing-at-point'.  With this type commands use syntax tables to
-navigate symbols and treesit definition to navigate lists.
+navigate symbols and treesit definitions to navigate lists.
 
 The `sexp' type uses the `sexp' thing defined in `treesit-thing-settings'.
-With this type commands use only the treesit definition of parser nodes,
+With this type commands use only the treesit definitions of parser nodes,
-without distinction between symbols and lists."
+without distinction between symbols and lists.  Since tree-sitter grammars
+could group node types in arbitrary ways, navigation by `sexp' might not
+match your expectations, and might produce different results in differnt
+treesit-based modes."
   (interactive "p")
   (if (not (treesit-thing-defined-p 'list (treesit-language-at (point))))
       (user-error "No `list' thing is defined in `treesit-thing-settings'")
@@ -3630,14 +3633,15 @@ predicate as described in `treesit-thing-settings'."
   (treesit--thing-sibling pos thing nil))
 
 (defun treesit-thing-at (pos thing &optional strict)
-  "Return the smallest THING enclosing POS.
+  "Return the smallest node enclosing POS for THING.
 
-The returned node, if non-nil, must enclose POS, i.e., its start
+The returned node, if non-nil, must enclose POS, i.e., its
-<= POS, its end > POS.  If STRICT is non-nil, the returned node's
+start <= POS, its end > POS.  If STRICT is non-nil, the returned
-start must < POS rather than <= POS.
+node's start must be < POS rather than <= POS.
 
-THING should be a thing defined in `treesit-thing-settings', or
+THING should be a thing defined in `treesit-thing-settings' for
-it can be a predicate described in `treesit-thing-settings'."
+the current buffer's major mode, or it can be a predicate
+described in `treesit-thing-settings'."
   (let* ((cursor (treesit-node-at pos))
          (iter-pred (lambda (node)
                       (and (treesit-node-match-p node thing t)
@@ -3789,13 +3793,14 @@ function is called recursively."
     (if (eq counter 0) pos nil)))
 
 (defun treesit-thing-at-point (thing tactic)
-  "Return the THING at point, or nil if none is found.
+  "Return the node for THING at point, or nil if no THING is found at point.
 
 THING can be a symbol, a regexp, a predicate function, and more;
-see `treesit-thing-settings' for details.
+for details, see `treesit-thing-settings' as defined by the
+current buffer's major mode.
 
-Return the top-level THING if TACTIC is `top-level'; return the
+Return the top-level node for THING if TACTIC is `top-level'; return
-smallest enclosing THING as POS if TACTIC is `nested'."
+the smallest node enclosing THING at point if TACTIC is `nested'."
 
   (let ((node (treesit-thing-at (point) thing)))
     (if (eq tactic 'top-level)

src/treesit.c

@@ -5193,13 +5193,16 @@ then in the system default locations for dynamic libraries, in that order.  */);
 	       doc:
 	       /* A list defining things.
 
-The value should be an alist of (LANGUAGE . DEFINITIONS), where
+The value should be defined by the major mode, and should be an alist
-LANGUAGE is a language symbol, and DEFINITIONS is a list of
+of the form (LANGUAGE . DEFINITIONS), where LANGUAGE is a language
+symbol and DEFINITIONS is a list whose elements are of the form
 
     (THING PRED)
 
-THING is a symbol representing the thing, like `defun', `sexp', or
+THING is a symbol representing the thing, like `defun', `defclass',
-`sentence'; PRED defines what kind of node can be qualified as THING.
+`sexp', `sentence', `comment', or any other symbol that is meaningful
+for the major mode; PRED defines what kind of node can be qualified
+as THING.
 
 PRED can be a regexp string that matches the type of the node; it can
 be a predicate function that takes the node as the sole argument and
@@ -5207,12 +5210,13 @@ returns t if the node is the thing, and nil otherwise; it can be a
 cons (REGEXP . FN), which is a combination of a regexp and a predicate
 function, and the node has to match both to qualify as the thing.
 
-PRED can also be recursively defined.  It can be (or PRED...), meaning
+PRED can also be recursively defined.  It can be:
-satisfying anyone of the inner PREDs qualifies the node; or (and
-PRED...) meaning satisfying all of the inner PREDs qualifies the node;
-or (not PRED), meaning not satisfying the inner PRED qualifies the node.
 
-There are two pre-defined predicates, `named' and `anonymous`.  They
+ (or PRED...), meaning satisfying any of the inner PREDs qualifies the node;
+ (and PRED...) meaning satisfying all of the inner PREDs qualifies the node;
+ (not PRED), meaning not satisfying the inner PRED qualifies the node.
+
+There are two pre-defined predicates, `named' and `anonymous'.  They
 match named nodes and anonymous nodes, respectively.
 
 Finally, PRED can refer to other THINGs defined in this list by using