media: doc: h264/hevc: Clarify _START_CODE_NONE meaning

As some hardware have a knob to enable or disable emulation prevention
bytes removal, driver writers ended up wondering what exactly the
START_CODE_NONE modes meant in this regards. This patch clarify what is
expected with a reference to the specification.

Reported-by: John Cox <jc@kynesim.co.uk>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

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

@@ -769,7 +769,9 @@ Stateless Codec Control ID
     * - ``V4L2_STATELESS_H264_START_CODE_NONE``
       - 0
       - Selecting this value specifies that H264 slices are passed
-        to the driver without any start code.
+        to the driver without any start code. The bitstream data should be
+        according to :ref:`h264` 7.3.1 NAL unit syntax, hence contains
+        emulation prevention bytes when required.
     * - ``V4L2_STATELESS_H264_START_CODE_ANNEX_B``
       - 1
       - Selecting this value specifies that H264 slices are expected

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

@@ -3719,7 +3719,9 @@ enum v4l2_mpeg_video_hevc_size_of_length_field -
     * - ``V4L2_MPEG_VIDEO_HEVC_START_CODE_NONE``
       - 0
       - Selecting this value specifies that HEVC slices are passed
-        to the driver without any start code.
+        to the driver without any start code. The bitstream data should be
+        according to :ref:`hevc` 7.3.1.1 General NAL unit syntax, hence
+        contains emulation prevention bytes when required.
     * - ``V4L2_MPEG_VIDEO_HEVC_START_CODE_ANNEX_B``
       - 1
       - Selecting this value specifies that HEVC slices are expected