mac80211: document only injected *_RADIOTAP_* flags

Not the internal flags but the radiotap flags are parsed when the monitor
injected frames are prepared for transmission. Thus the documentation
should only document these.

Reported-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Fixes: dfdfc2be ("mac80211: Parse legacy and HT rate in injected frames")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>