[media] v4l: Document timestamp buffer flag behaviour

Timestamp buffer flags are constant at the moment. Document them so that 1)
they're always valid and 2) not changed by the drivers. This leaves room to
extend the functionality later on if needed.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>