Remove mention of the compatibility option 'q', which is
intentionally undocumented and its only purpose is that
we do not bail out when used as a drop-in replacement of
a different implementation.

PR:		docs/184550
MFC after:	2 weeks
Add some PLCP thresholds from Linux iwlwifi driver in preparation for
working on some RF tuning issues.

The linux iwlwifi driver has these thresholds which they use to see
if there are PLCP errors over a certain interval.  If they hit this,
they trigger a single-channel (different from active channels!)
scan to retune the RF front-end.