media: extended-controls.rst: add note to the MPEG2 state controls

Add a note mentioning that these two controls are not part of the
public API while they still stabilizing.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/v4l/extended-controls.rst

@@ -1505,6 +1505,11 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
     configuring a stateless hardware decoding pipeline for MPEG-2.
     The bitstream parameters are defined according to :ref:`mpeg2part2`.
 
+    .. note::
+
+       This compound control is not yet part of the public kernel API and
+       it is expected to change.
+
 .. c:type:: v4l2_ctrl_mpeg2_slice_params
 
 .. cssclass:: longtable
@@ -1625,6 +1630,11 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
     Specifies quantization matrices (as extracted from the bitstream) for the
     associated MPEG-2 slice data.
 
+    .. note::
+
+       This compound control is not yet part of the public kernel API and
+       it is expected to change.
+
 .. c:type:: v4l2_ctrl_mpeg2_quantization
 
 .. cssclass:: longtable