ChangeLog:
	* manual/stdio.texi (Customizing printf): Add a note to disable
	built-in handling.

diff --git a/manual/stdio.texi b/manual/stdio.texi
index 38be236..d945955 100644
--- a/manual/stdio.texi
+++ b/manual/stdio.texi
@@ -2963,7 +2963,11 @@ The facilities of this section are declared in the header file
 
 @strong{Portability Note:} The ability to extend the syntax of
 @code{printf} template strings is a GNU extension.  ISO standard C has
-nothing similar.
+nothing similar.  When using the GNU C compiler or any other compiler
+that interprets calls to standard I/O functions according to the rules
+of the language standard it is necessary to disable such handling by
+the appropriate compiler option.  Otherwise the behavior of a program
+that relies on the extension is undefined.
 
 @node Registering New Conversions
 @subsection Registering New Conversions