* doc/lispref/debugging.texi (Error Debugging): Mention debug-on-message.

* etc/NEWS: Related markup.

doc/lispref/ChangeLog

 2012-11-11  Glenn Morris  <rgm@gnu.org>
 
+	* debugging.texi (Error Debugging): Mention debug-on-message.
+
 	* control.texi (Signaling Errors):
 	* debugging.texi (Error Debugging):
 	* errors.texi (Standard Errors): Add user-error.

doc/lispref/debugging.texi

@@ -163,6 +163,14 @@ supported values correspond to the signals @code{SIGUSR1} and
 @code{inhibit-quit} is set and Emacs is not otherwise responding.
 @end defopt
 
+@cindex message, finding what causes a particular message
+@defvar debug-on-message
+If you set @code{debug-on-message} to a regular expression,
+Emacs will enter the debugger if it displays a matching message in the
+echo area.  For example, this can be useful when trying to find the
+cause of a particular message.
+@end defvar
+
   To debug an error that happens during loading of the init
 file, use the option @samp{--debug-init}.  This binds
 @code{debug-on-error} to @code{t} while loading the init file, and

etc/NEWS

@@ -806,6 +806,7 @@ These do not trigger the debugger.
 
 *** New option `debugger-bury-or-kill'.
 
++++
 *** Set `debug-on-message' to enter the debugger when a certain
 message is displayed in the echo area.  This can be useful when trying
 to work out which code is doing something.