[media] Documentation: media: Clarify the VIDIOC_CREATE_BUFS format requirements

The VIDIOC_CREATE_BUFS ioctl takes a format argument that must contain a
valid format supported by the driver. Clarify the documentation.

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