Input: add keyboard notifier documentation

Document the keyboard notifier.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
SIgned-off-by: Dmitry Torokhov <dtor@mail.ru>