media: uapi: h264: clarify num_ref_idx_l[01]_(default_)active fields

Since the uapi does not contain the num_ref_idx_active_override_flag,
drivers for decoders that do not parse slices themselves don't know
how to choose between the num_ref_idx_l[01]_default_active and the
num_ref_idx_l[01]_active override fields.

Instead, userspace must set the override fields to the default values
if the slice does not have the num_ref_idx_active_override flag set.
The drivers will then always enable the override internally and ignore
the default fields completely.

Clarify this requirement in the API documentation.
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

@@ -1820,10 +1820,12 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
       -
     * - __u8
       - ``num_ref_idx_l0_active_minus1``
-      -
+      - If num_ref_idx_active_override_flag is not set, this field must be
+        set to the value of num_ref_idx_l0_default_active_minus1.
     * - __u8
       - ``num_ref_idx_l1_active_minus1``
-      -
+      - If num_ref_idx_active_override_flag is not set, this field must be
+        set to the value of num_ref_idx_l1_default_active_minus1.
     * - __u32
       - ``slice_group_change_cycle``
       -