wifi: mac80211: add documentation for amsdu_mesh_control

This documentation wasn't added in the original patch,
add it now.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 6e4c0d04

 ("wifi: mac80211: add a workaround for receiving non-standard mesh A-MSDU")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>