[media] media: vb2: Clarify queue_setup() and buf_prepare() usage documentation

Explain how the two operations must handle formats and validate buffer
sizes when used with VIDIOC_CREATE_BUFS.

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>