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>