Improve documentation of several functions in seq.el

* doc/lispref/sequences.texi (Sequence Functions):
* lisp/emacs-lisp/seq.el (seq-contains):
(seq-contains-p):
(seq-set-equal-p):
(seq-position):
(seq-union):
(seq-intersection):
(seq-difference): Use more standard wording in the docstrings
(bug#57561).

doc/lispref/sequences.texi

@@ -863,7 +863,7 @@ arguments to use instead of the default @code{equal}.
 @end defun
 
 @defun seq-position sequence elt &optional function
-  This function returns the index of the first element in
+  This function returns the (zero-based) index of the first element in
 @var{sequence} that is equal to @var{elt}.  If the optional argument
 @var{function} is non-@code{nil}, it is a function of two arguments to
 use instead of the default @code{equal}.

lisp/emacs-lisp/seq.el

@@ -409,7 +409,7 @@ found or not."
 
 (cl-defgeneric seq-contains (sequence elt &optional testfn)
   "Return the first element in SEQUENCE that is equal to ELT.
-Equality is defined by TESTFN if non-nil or by `equal' if nil."
+Equality is defined by the function TESTFN, which defaults to `equal'."
   (declare (obsolete seq-contains-p "27.1"))
   (seq-some (lambda (e)
               (when (funcall (or testfn #'equal) elt e)
@@ -418,7 +418,7 @@ Equality is defined by TESTFN if non-nil or by `equal' if nil."
 
 (cl-defgeneric seq-contains-p (sequence elt &optional testfn)
   "Return non-nil if SEQUENCE contains an element equal to ELT.
-Equality is defined by TESTFN if non-nil or by `equal' if nil."
+Equality is defined by the function TESTFN, which defaults to `equal'."
     (catch 'seq--break
       (seq-doseq (e sequence)
         (let ((r (funcall (or testfn #'equal) e elt)))
@@ -429,14 +429,14 @@ Equality is defined by TESTFN if non-nil or by `equal' if nil."
 (cl-defgeneric seq-set-equal-p (sequence1 sequence2 &optional testfn)
   "Return non-nil if SEQUENCE1 and SEQUENCE2 contain the same elements.
 This does not depend on the order of the elements.
-Equality is defined by TESTFN if non-nil or by `equal' if nil."
+Equality is defined by the function TESTFN, which defaults to `equal'."
   (and (seq-every-p (lambda (item1) (seq-contains-p sequence2 item1 testfn)) sequence1)
        (seq-every-p (lambda (item2) (seq-contains-p sequence1 item2 testfn)) sequence2)))
 
 ;;;###autoload
 (cl-defgeneric seq-position (sequence elt &optional testfn)
-  "Return the index of the first element in SEQUENCE that is equal to ELT.
-Equality is defined by TESTFN if non-nil or by `equal' if nil."
+  "Return the (zero-based) index of the first element in SEQUENCE equal to ELT.
+Equality is defined by the function TESTFN, which defaults to `equal'."
   (let ((index 0))
     (catch 'seq--break
       (seq-doseq (e sequence)
@@ -502,7 +502,7 @@ negative integer or 0, nil is returned."
 ;;;###autoload
 (cl-defgeneric seq-union (sequence1 sequence2 &optional testfn)
   "Return a list of all elements that appear in either SEQUENCE1 or SEQUENCE2.
-Equality is defined by TESTFN if non-nil or by `equal' if nil."
+Equality is defined by the function TESTFN, which defaults to `equal'."
   (let* ((accum (lambda (acc elt)
                   (if (seq-contains-p acc elt testfn)
                       acc
@@ -514,7 +514,7 @@ Equality is defined by TESTFN if non-nil or by `equal' if nil."
 ;;;###autoload
 (cl-defgeneric seq-intersection (sequence1 sequence2 &optional testfn)
   "Return a list of the elements that appear in both SEQUENCE1 and SEQUENCE2.
-Equality is defined by TESTFN if non-nil or by `equal' if nil."
+Equality is defined by the function TESTFN, which defaults to `equal'."
   (seq-reduce (lambda (acc elt)
                 (if (seq-contains-p sequence2 elt testfn)
                     (cons elt acc)
@@ -524,7 +524,7 @@ Equality is defined by TESTFN if non-nil or by `equal' if nil."
 
 (cl-defgeneric seq-difference (sequence1 sequence2 &optional testfn)
   "Return a list of the elements that appear in SEQUENCE1 but not in SEQUENCE2.
-Equality is defined by TESTFN if non-nil or by `equal' if nil."
+Equality is defined by the function TESTFN, which defaults to `equal'."
   (seq-reduce (lambda (acc elt)
                 (if (seq-contains-p sequence2 elt testfn)
                     acc