media: uapi: h264: clarify dec_ref_pic_marking_bit_size fields

Since dec_ref_pic_marking_bit_size is not a syntax element
itself, explicitly state that this is the size in bits of
the dec_ref_pic_marking() syntax element contained in the
slice.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/v4l/ext-ctrls-codec.rst

@@ -1796,7 +1796,7 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
       -
     * - __u32
       - ``dec_ref_pic_marking_bit_size``
-      -
+      - Size in bits of the dec_ref_pic_marking() syntax element.
     * - __u32
       - ``pic_order_cnt_bit_size``
       -