(Format Conversion Round-Trip): Mention nil regexp.

doc/lispref/ChangeLog

@@ -2,6 +2,8 @@
 
 	* display.texi (Fontsets): Fix typo.
 
+	* files.texi (Format Conversion Round-Trip): Mention nil regexp.
+
 2009-08-19  Stefan Monnier  <monnier@iro.umontreal.ca>
 
 	* processes.texi (Asynchronous Processes): Adjust arglist of

doc/lispref/files.texi

@@ -2901,7 +2901,7 @@ A documentation string for the format.
 
 @item regexp
 A regular expression which is used to recognize files represented in
-this format.
+this format.  If @code{nil}, the format is never applied automatically.
 
 @item from-fn
 A shell command or function to decode data in this format (to convert