media: uapi: h264: Clarify pic_order_cnt_bit_size field

Since pic_order_cnt_bit_size is not a syntax element itself, explicitly
state that it is the total size in bits of the pic_order_cnt_lsb,
delta_pic_order_cnt_bottom, delta_pic_order_cnt[0], and
delta_pic_order_cnt[1] syntax elements contained in the slice.

[Ezequiel: rebase]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: Jonas Karlman <jonas@kwiboo.se>
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.rst

@@ -1815,7 +1815,9 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
       - Size in bits of the dec_ref_pic_marking() syntax element.
     * - __u32
       - ``pic_order_cnt_bit_size``
-      -
+      - Combined size in bits of the picture order count related syntax
+        elements: pic_order_cnt_lsb, delta_pic_order_cnt_bottom,
+        delta_pic_order_cnt0, and delta_pic_order_cnt1.
     * - __u8
       - ``cabac_init_idc``
       -