cfg80211/mac80211: clarify RSSI CQM reporting requirements

The previous patch changed mac80211 to always report an event
after a CQM RSSI reconfiguration. Document that as expected
behaviour in both the cfg80211 and mac80211 API.

Currently, iwlmvm already implements that behaviour; the other
drivers implementing CQM RSSI events may have to be changed.

This behaviour lets userspace know what the current state is
without relying on querying the data which is racy.

Reviewed-by: Sharon, Sara <sara.sharon@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>