rfkill: improve documentation for kernel drivers

Improve the documentation of how to use the rfkill class in kernel drivers,
based on the doubts that came up in a thread in linux-wireless.
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>