my.cnf: mention that config files must be *.cnf

It took me a long time to debug why my configs were not being loaded,
and judging from online discussions I'm not the only one. Make the
comment in the default my.cnf a bit more helpful.

The !includedir directive is implemented in mysys/my_default.c.

- f_extensions[] is a list of file extensions. It includes .ini and .cnf
  on Windows, and only .cnf on all other platforms.
- search_default_file_with_ext() contains the !includedir directive. It
  filters files in the directory to those matching f_extensions[].

This file should only be applicable on Unix-like platforms, so only
files with the .cnf extension are read.

Closes #1485

support-files/rpm/my.cnf

@@ -5,7 +5,7 @@
 [client-server]
 
 #
-# include all files from the config directory
+# include *.cnf from the config directory
 #
 !includedir /etc/my.cnf.d