cfg80211: Improve connection quality maintenance docs in nl80211.h

In nl80211.h, be a little more elaborate in the docs for the definitions
NL80211_ATTR_CQM_RSSI_THOLD and NL80211_ATTR_CQM_RSSI_HYST.

Reported-by: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>