mac80211: add some missing kernel-doc

Add a few kernel-doc descriptions that were missed
during development.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/mac80211/sta_info.h b/net/mac80211/sta_info.h
index 3bb24a1..525ce50 100644
--- a/net/mac80211/sta_info.h
+++ b/net/mac80211/sta_info.h
@@ -278,6 +278,8 @@
  * @sta: station information we share with the driver
  * @sta_state: duplicates information about station state (for debug)
  * @beacon_loss_count: number of times beacon loss has triggered
+ * @supports_40mhz: tracks whether the station advertised 40 MHz support
+ *	as we overwrite its HT parameters with the currently used value
  */
 struct sta_info {
 	/* General information, mostly static */