mac80211: add docs for ssn in struct tid_ampdu_tx

As pointed out by Stephen, add the missing docs.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20211129091948.1327ec82beab.Iecc5975406a3028d35c65ff8d2dec31a693888d3@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.h

@@ -176,6 +176,7 @@ struct sta_info;
  * @failed_bar_ssn: ssn of the last failed BAR tx attempt
  * @bar_pending: BAR needs to be re-sent
  * @amsdu: support A-MSDU withing A-MDPU
+ * @ssn: starting sequence number of the session
  *
  * This structure's lifetime is managed by RCU, assignments to
  * the array holding it must hold the aggregation mutex.