Improve documentation of regexp search for categories.

 doc/lispref/searching.texi (Regexp Backslash): Document how to display
 existing categories.  Mention the possibility of adding
 categories, and add an xref to where this is described.  Add an
 index entry.
 doc/lispref/syntax.texi (Categories): Add an example of defining a new
 category and category table.

1 files changed, 5 insertions, 1 deletions

diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi
index 6272301dbb4..fe7c805c6f7 100644
--- a/doc/lispref/searching.texi
+++ b/doc/lispref/searching.texi
@@ -717,11 +717,15 @@ the characters that stand for them.
 @cindex @samp{\S} in regexp
 matches any character whose syntax is not @var{code}.
 
+@cindex category, regexp search for
 @item \c@var{c}
 matches any character whose category is @var{c}.  Here @var{c} is a
 character that represents a category: thus, @samp{c} for Chinese
 characters or @samp{g} for Greek characters in the standard category
-table.
+table.  You can see the list of all the currently defined categories
+with @kbd{M-x describe-categories @key{RET}}.  You can also define
+your own categories in addition to the standard ones using the
+@code{define-category} function (@pxref{Categories}).
 
 @item \C@var{c}
 matches any character whose category is not @var{c}.