cfg80211: Improve Connect/Associate command documentation

The roaming cases for the Connect command were not fully covered and
neither Connect nor Associate command uses of the prev_bssid parameter
were very clear. Add details to describe how the prev_bssid argument is
supposed to be used and when the driver should use association or
reassociation.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>