Index: doc/lispref/searching.texi
===================================================================
RCS file: /sources/emacs/emacs/doc/lispref/searching.texi,v
retrieving revision 1.11
diff -u -r1.11 searching.texi
--- doc/lispref/searching.texi	8 Jan 2009 05:29:01 -0000	1.11
+++ doc/lispref/searching.texi	3 Mar 2009 20:48:30 -0000
@@ -1204,6 +1204,12 @@
 can't avoid another intervening search, you must save and restore the
 match data around it, to prevent it from being overwritten.
 
+  Notice that all functions are allowed to overwrite the match data
+unless they're explicitly documented not to do so. A consequence is that
+functions that are run implictly in the background (@pxref{Timers}, and
+@ref{Idle Timers}) should likely save and restore the match data
+explicitly.
+
 @menu
 * Replacing Match::	  Replacing a substring that was matched.
 * Simple Match Data::     Accessing single items of match data,