nl80211: document uapi for CMD_FRAME_WAIT_CANCEL

Commit 1c38c7f2 ("nl80211: send event when CMD_FRAME duration
expires") added the possibility of NL80211_CMD_FRAME_WAIT_CANCEL
being sent whenever the off-channel wait time associated with a
CMD_FRAME completes.  Document this in the uapi/linux/nl80211.h file.
Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Link: https://lore.kernel.org/r/20190722113312.14031-1-denkenz@gmail.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>

include/uapi/linux/nl80211.h

@@ -657,7 +657,9 @@
  *	is used during CSA period.
  * @NL80211_CMD_FRAME_WAIT_CANCEL: When an off-channel TX was requested, this
  *	command may be used with the corresponding cookie to cancel the wait
- *	time if it is known that it is no longer necessary.
+ *	time if it is known that it is no longer necessary.  This command is
+ *	also sent as an event whenever the driver has completed the off-channel
+ *	wait time.
  * @NL80211_CMD_ACTION: Alias for @NL80211_CMD_FRAME for backward compatibility.
  * @NL80211_CMD_FRAME_TX_STATUS: Report TX status of a management frame
  *	transmitted with %NL80211_CMD_FRAME. %NL80211_ATTR_COOKIE identifies