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: NLuis de Bethencourt <luisbg@osg.samsung.com>
[fix description]
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>