mac80211: fix comment regarding aggregation buf_size

The description for buf_size was misleading and
just said you couldn't TX larger aggregates, but
of course you can't TX aggregates in a way that
would exceed the window either, which is possible
even if the aggregates are shorter than that.

Expand the description, thanks to Emmanuel for
explaining this to me.

Cc: Emmanuel Grumbach <egrumbach@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -1753,8 +1753,19 @@ enum ieee80211_ampdu_mlme_action {
  * 	that TX/RX_STOP can pass NULL for this parameter.
  *	The @buf_size parameter is only valid when the action is set to
  *	%IEEE80211_AMPDU_TX_OPERATIONAL and indicates the peer's reorder
- *	buffer size (number of subframes) for this session -- aggregates
- *	containing more subframes than this may not be transmitted to the peer.
+ *	buffer size (number of subframes) for this session -- the driver
+ *	may neither send aggregates containing more subframes than this
+ *	nor send aggregates in a way that lost frames would exceed the
+ *	buffer size. If just limiting the aggregate size, this would be
+ *	possible with a buf_size of 8:
+ *	 - TX: 1.....7
+ *	 - RX:  2....7 (lost frame #1)
+ *	 - TX:        8..1...
+ *	which is invalid since #1 was now re-transmitted well past the
+ *	buffer size of 8. Correct ways to retransmit #1 would be:
+ *	 - TX:       1 or 18 or 81
+ *	Even "189" would be wrong since 1 could be lost again.
+ *
  *	Returns a negative error code on failure.
  *	The callback can sleep.
  *