[media] DocBook v4l2: update the G/S_EDID documentation

Document that it is now possible to call G/S_EDID from video nodes, not
just sub-device nodes. Add a note that -EINVAL will be returned if
the pad does not support EDIDs.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>