rfkill: document rw rfkill switches and clarify input subsystem interactions

Rework the documentation so as to make sure driver writers understand
exactly where the boundaries are for input drivers related to rfkill
switches, buttons and keys, and rfkill class drivers.

Also fix a small error in the documentation: setting the state of a normal
instance of the rfkill class does not affect the state of any other devices
(unless they are tied by firmware/hardware somehow).
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Cc: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: John W. Linville <linville@tuxdriver.com>