wireless: update cfg80211 kconfig entry

cfg80211 is now *the* wireless configuration API. Lets also
give a little explanation as to what it is and refer people to
the wireless wiki for more information.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>