doc: media: Document VP9 reference_mode miss-placement

The reference_mode derived syntax is part of the compressed headers and
should have been moved into the corresponding control structure. Document
this mistake. The value can be set to 0 if the driver does not require
compressed headers information.
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: fix small typo in comment]

Documentation/userspace-api/media/v4l/ext-ctrls-codec-stateless.rst

@@ -1692,7 +1692,12 @@ See section '7.3.1 Tx mode semantics' of the :ref:`vp9` specification for more d
     * - __u8
       - ``reference_mode``
       - Specifies the type of inter prediction to be used. See
-        :ref:`Reference Mode<vp9_reference_mode>` for more details.
+        :ref:`Reference Mode<vp9_reference_mode>` for more details. Note that
+	this is derived as part of the compressed header parsing process and
+	for this reason should have been part of
+	:c:type: `v4l2_ctrl_vp9_compressed_hdr` optional control. It is safe to
+	set this value to zero if the driver does not require compressed
+	headers.
     * - __u8
       - ``reserved[7]``
       - Applications and drivers must set this to zero.