mac80211: fix two missing documentation entries

Add documentation for two struct entries that was missing.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200424123945.6b23a26ab5e7.I664440ab5f33442df8103253bf5b9fe84be8d58c@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>