nl80211: clarify NL80211_ATTR_SCHED_SCAN_DELAY usage with net-detect

In this attribute's documentation, it was not clear whether the delay
started counting when WoWLAN net-detect was enabled or when the system
was suspended.  The correct answer is that it starts when the system
suspends (which is when, in practice, the scan is scheduled).  Clarify
that in the nl80211.h documentation.

Suggested-by: Samuel Tan <samueltan@google.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>