[media] v4l: Improve sub-device documentation for pad ops

Document that format related configuration is done through pad ops in case
the driver does use the media framework.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>