mac80211: add doc for RX_FLAG_DUP_VALIDATED flag

Add documentation for the flag for duplication check.

Fixes the following warning when running make htmldocs:
warning: Enum value 'RX_FLAG_DUP_VALIDATED' not described in enum 'mac80211_rx_flags'
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
[fix description]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -1001,6 +1001,8 @@ ieee80211_tx_info_clear_status(struct ieee80211_tx_info *info)
  *	flag indicates that the PN was verified for replay protection.
  *	Note that this flag is also currently only supported when a frame
  *	is also decrypted (ie. @RX_FLAG_DECRYPTED must be set)
+ * @RX_FLAG_DUP_VALIDATED: The driver should set this flag if it did
+ *	de-duplication by itself.
  * @RX_FLAG_FAILED_FCS_CRC: Set this flag if the FCS check failed on
  *	the frame.
  * @RX_FLAG_FAILED_PLCP_CRC: Set this flag if the PCLP check failed on