media: v4l2-subdev: Clearly document that the crop API won't be extended

media: v4l2-subdev: Clearly document that the crop API won't be extended

The V4L2 subdev crop API has been marked as obsolete, deprecated by the
selection API. Despite this, it has recently been extended with streams
support. In hindsight this was a mistake. Make sure it doesn't happen
again by clearly documenting that no new extensions will be accepted.

Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>