Document this new flag.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Martin Bugge <marbugge@cisco.com>
Cc: Mats Randgaard <mats.randgaard@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>