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.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>