mac80211: document status.freq restrictions

It's not always necessary to set the status.freq field, for example
when this would be an expensive calculation. It must be set for all
management frames (as they might be reported to userspace), but for
data frames it's not really required. Document this.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -1108,6 +1108,8 @@ enum mac80211_rx_vht_flags {
  *	it but can store it and pass it back to the driver for synchronisation
  * @band: the active band when this frame was received
  * @freq: frequency the radio was tuned to when receiving this frame, in MHz
+ *	This field must be set for management frames, but isn't strictly needed
+ *	for data (other) frames - for those it only affects radiotap reporting.
  * @signal: signal strength when receiving this frame, either in dBm, in dB or
  *	unspecified depending on the hardware capabilities flags
  *	@IEEE80211_HW_SIGNAL_*