[media] v4l: Unify selection flags documentation

As for the selection targets, the selection flags are now the same on V4L2
and V4L2 subdev interfaces. Also document them so.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>