mac80211: extend sta kdoc - explain when they are added

 * When the insertion fails (sta_info_insert()) returns non-zero), the

 * structure will have been freed by sta_info_insert()!

 *

 * sta entries are added by mac80211 when you establish a link with a

 * peer. This means different things for the different type of interfaces

 * we support. For a regular station this mean we add the AP sta when we

 * receive an assocation response from the AP. For IBSS this occurs when

 * we receive a probe response or a beacon from target IBSS network. For

 * WDS we add the sta for the peer imediately upon device open. When using

 * AP mode we add stations for each respective station upon request from

 * userspace through nl80211.

 *

 * Because there are debugfs entries for each station, and adding those

 * must be able to sleep, it is also possible to "pin" a station entry,

 * that means it can be removed from the hash table but not be freed.