The specification mentions that type can be V4L2_BUF_TYPE_VIDEO_CAPTURE,
but the v4l2 core implementation also allows the _MPLANE variant.

Document this.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>