mac80211: document powersaving/beacon filter future

Document what mac80211 will do in the future to help save power.
We're not quite there yet, but a plan helps. Also, while at it,
fix the docs wrt. multicast traffic.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>