declare newmail_hook option stable

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -302,8 +302,6 @@ remoterepository = RemoteExample
 #
 # This example plays a sound file of your chosing when new mail arrives.
 #
-# This feature is experimental.
-#
 #newmail_hook = lambda: os.system("cvlc --play-and-stop --play-and-exit /path/to/sound/file.mp3" +
 #				 " > /dev/null 2>&1")