Document that FcConfigGetFonts returns the internal fontset (bug 13197)

FcConfigGetFonts returns the internal font set used by the library which
must not be freed by the application or 'bad things' will happen.

doc/fcconfig.fncs

@@ -149,7 +149,8 @@ cache file for a font directory.
 @PURPOSE@	Get config font set
 @DESC@
 Returns one of the two sets of fonts from the configuration as specified
-by <parameter>set</parameter>.
+by <parameter>set</parameter>. This font set is owned by the library and must
+not be freed.
 @@
 
 @RET@           FcBlanks *