Clarify the documentation of back-references in replacements

* doc/emacs/search.texi (Regexp Replace): Clarify that \D starts
with \1, not \0.

* lisp/replace.el (query-replace-regexp)
(query-replace-regexp-eval, replace-regexp): Doc fix (Bug#23884)

doc/emacs/search.texi

@@ -1356,12 +1356,12 @@ Replace every match for @var{regexp} with @var{newstring}.
 it can refer to all or part of what is matched by the @var{regexp}.
 @samp{\&} in @var{newstring} stands for the entire match being
 replaced.  @samp{\@var{d}} in @var{newstring}, where @var{d} is a
-digit, stands for whatever matched the @var{d}th parenthesized
+digit starting from 1, stands for whatever matched the @var{d}th
-grouping in @var{regexp}.  (This is called a ``back reference''.)
+parenthesized grouping in @var{regexp}.  (This is called a ``back
-@samp{\#} refers to the count of replacements already made in this
+reference''.)  @samp{\#} refers to the count of replacements already
-command, as a decimal number.  In the first replacement, @samp{\#}
+made in this command, as a decimal number.  In the first replacement,
-stands for @samp{0}; in the second, for @samp{1}; and so on.  For
+@samp{\#} stands for @samp{0}; in the second, for @samp{1}; and so on.
-example,
+For example,
 
 @example
 M-x replace-regexp @key{RET} c[ad]+r @key{RET} \&-safe @key{RET}

lisp/replace.el

@@ -391,9 +391,10 @@ replace backward.
 
 Fourth and fifth arg START and END specify the region to operate on.
 
-In TO-STRING, `\\&' stands for whatever matched the whole of REGEXP,
+In TO-STRING, `\\&' or `\\0' stands for whatever matched the whole of
-and `\\=\\N' (where N is a digit) stands for
+REGEXP, and `\\=\\N' (where N is a digit) stands for whatever matched
-whatever what matched the Nth `\\(...\\)' in REGEXP.
+the Nth `\\(...\\)' (1-based) in REGEXP.  The `\\(...\\)' groups are
+counted from 1.
 `\\?' lets you edit the replacement text in the minibuffer
 at the given position for each replacement.
 
@@ -451,7 +452,9 @@ If the result of TO-EXPR is not a string, it is converted to one using
 
 For convenience, when entering TO-EXPR interactively, you can use `\\&' or
 `\\0' to stand for whatever matched the whole of REGEXP, and `\\N' (where
-N is a digit) to stand for whatever matched the Nth `\\(...\\)' in REGEXP.
+N is a digit) to stand for whatever matched the Nth `\\(...\\)' (1-based)
+in REGEXP.
+
 Use `\\#&' or `\\#N' if you want a number instead of a string.
 In interactive use, `\\#' in itself stands for `replace-count'.
 
@@ -635,9 +638,9 @@ replace backward.
 
 Fourth and fifth arg START and END specify the region to operate on.
 
-In TO-STRING, `\\&' stands for whatever matched the whole of REGEXP,
+In TO-STRING, `\\&' or `\\0' stands for whatever matched the whole of
-and `\\=\\N' (where N is a digit) stands for
+REGEXP, and `\\=\\N' (where N is a digit) stands for
-whatever what matched the Nth `\\(...\\)' in REGEXP.
+whatever matched the Nth `\\(...\\)' (1-based) in REGEXP.
 `\\?' lets you edit the replacement text in the minibuffer
 at the given position for each replacement.