[media] v4l: Clean up sub-device format documentation

The sub-device format documentation documented scaling configuration through
formats. Instead the compose selection rectangle is elsewhere documented to
be used for the purpose. Remove scaling related part of the documentation.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>