cfg80211: add mesh join/leave callback docs

When I made the patch to add mesh join/leave I
didn't pay attention to docs because it was a
proof of concept, and then when we actually did
merge it I forgot -- add docs now.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>