(prefer-coding-system): Add an example

Fixes: debbugs:4869

lisp/ChangeLog

 2011-07-13  Lars Magne Ingebrigtsen  <larsi@gnus.org>
 
+	* international/mule-cmds.el (prefer-coding-system): Add an
+	example (bug#4869).
+
 	* progmodes/etags.el (tags-search): Document `file-list-form'
 	(bug#4731).
 

lisp/international/mule-cmds.el

@@ -397,7 +397,11 @@ If CODING-SYSTEM specifies a certain type of EOL conversion, the coding
 systems set by this function will use that type of EOL conversion.
 
 A coding system that requires automatic detection of text+encoding
-\(e.g. undecided, unix) can't be preferred."
+\(e.g. undecided, unix) can't be preferred.
+
+To prefer, for instance, utf-8, say the following:
+
+  \(prefer-coding-system 'utf-8)"
   (interactive "zPrefer coding system: ")
   (if (not (and coding-system (coding-system-p coding-system)))
       (error "Invalid coding system `%s'" coding-system))