media: pixfmt: Add H264_SLICE_RAW format documentation

The H264_SLICE_RAW format introduced before is meant for stateless
decoders that will need the H264 parsed slice data without the start code.

Let's document it.
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/v4l/pixfmt-compressed.rst

@@ -52,6 +52,31 @@ Compressed Formats
       - ``V4L2_PIX_FMT_H264_MVC``
       - 'M264'
       - H264 MVC video elementary stream.
+    * .. _V4L2-PIX-FMT-H264-SLICE-RAW:
+
+      - ``V4L2_PIX_FMT_H264_SLICE_RAW``
+      - 'S264'
+      - H264 parsed slice data, without the start code and as
+	extracted from the H264 bitstream.  This format is adapted for
+	stateless video decoders that implement an H264 pipeline
+	(using the :ref:`mem2mem` and :ref:`media-request-api`).
+	Metadata associated with the frame to decode are required to
+	be passed through the ``V4L2_CID_MPEG_VIDEO_H264_SPS``,
+	``V4L2_CID_MPEG_VIDEO_H264_PPS``,
+	``V4L2_CID_MPEG_VIDEO_H264_SCALING_MATRIX``,
+	``V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS`` and
+	``V4L2_CID_MPEG_VIDEO_H264_DECODE_PARAMS`` controls.  See the
+	:ref:`associated Codec Control IDs <v4l2-mpeg-h264>`.  Exactly
+	one output and one capture buffer must be provided for use
+	with this pixel format. The output buffer must contain the
+	appropriate number of macroblocks to decode a full
+	corresponding frame to the matching capture buffer.
+
+	.. note::
+
+	   This format is not yet part of the public kernel API and it
+	   is expected to change.
+
     * .. _V4L2-PIX-FMT-H263:
 
       - ``V4L2_PIX_FMT_H263``