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 <[e-mail]>
Signed-off-by: Laurent Pinchart <[e-mail]>
Acked-by: Sakari Ailus <[e-mail]>
Reviewed-by: Tomi Valkeinen <[e-mail]>
Signed-off-by: Sakari Ailus <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

Documentation/userspace-api/media/v4l/vidioc-subdev-g-crop.rst

···
 
 .. note::
 
-    This is an :ref:`obsolete` interface and may be removed
-    in the future. It is superseded by
-    :ref:`the selection API <VIDIOC_SUBDEV_G_SELECTION>`.
+    This is an :ref:`obsolete` interface and may be removed in the future. It is
+    superseded by :ref:`the selection API <VIDIOC_SUBDEV_G_SELECTION>`. No new
+    extensions to the :c:type:`v4l2_subdev_crop` structure will be accepted.
 
 To retrieve the current crop rectangle applications set the ``pad``
 field of a struct :c:type:`v4l2_subdev_crop` to the

include/uapi/linux/v4l2-subdev.h

···
  * @rect: pad crop rectangle boundaries
  * @stream: stream number, defined in subdev routing
  * @reserved: drivers and applications must zero this array
+ *
+ * The subdev crop API is an obsolete interface and may be removed in the
+ * future. It is superseded by the selection API. No new extensions to this
+ * structure will be accepted.
  */
 struct v4l2_subdev_crop {
 	__u32 which;