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>