Refine documentation of `looking-back'.

* lisp/subr.el (looking-back): Do not recommend using looking-back.  (bug#5689)
* doc/lispref/searching.texi (Regexp Search): Refine.

doc/lispref/ChangeLog

+2013-09-29  Xue Fuqiao  <xfq.free@gmail.com>
+
+	* searching.texi (Regexp Search): Refine.
+
 2013-09-22  Xue Fuqiao  <xfq.free@gmail.com>
 
 	* nonascii.texi (Default Coding Systems): Typo fix.

doc/lispref/searching.texi

@@ -1146,13 +1146,7 @@ implemented by searching backwards from point for a match that ends at
 point.  That can be quite slow if it has to search a long distance.
 You can bound the time required by specifying @var{limit}, which says
 not to search before @var{limit}.  In this case, the match that is
-found must begin at or after @var{limit}.
-
-If @var{greedy} is non-@code{nil}, this function extends the match
-backwards as far as possible, stopping when a single additional
-previous character cannot be part of a match for regexp.  When the
-match is extended, its starting position is allowed to occur before
-@var{limit}.
+found must begin at or after @var{limit}.  Here's an example:
 
 @example
 @group
@@ -1168,6 +1162,12 @@ comes back" twice.
 @end group
 @end example
 
+If @var{greedy} is non-@code{nil}, this function extends the match
+backwards as far as possible, stopping when a single additional
+previous character cannot be part of a match for regexp.  When the
+match is extended, its starting position is allowed to occur before
+@var{limit}.
+
 @c http://debbugs.gnu.org/5689
 As a general recommendation, try to avoid using @code{looking-back}
 wherever possible, since it is slow.  For this reason, there are no

lisp/ChangeLog

+2013-09-29  Xue Fuqiao  <xfq.free@gmail.com>
+
+	* subr.el (looking-back): Do not recommend using looking-back.
+
 2013-09-28  Alan Mackenzie  <acm@muc.de>
 
 	Fix indentation/fontification of Java enum with "implements".

lisp/subr.el

@@ -3482,7 +3482,10 @@ If GREEDY is non-nil, extend the match backwards as far as
 possible, stopping when a single additional previous character
 cannot be part of a match for REGEXP.  When the match is
 extended, its starting position is allowed to occur before
-LIMIT."
+LIMIT.
+
+As a general recommendation, try to avoid using `looking-back'
+wherever possible, since it is slow."
   (let ((start (point))
 	(pos
 	 (save-excursion