mac80211: update aggregation documentation

Even before the recent changes, the documentation
for TX aggregation was somewhat out of date. Update
it and also add documentation for the RX side.

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

diff --cc net/mac80211/agg-rx.c
Simple merge

diff --cc net/mac80211/agg-tx.c
Simple merge