doc: media: Document VP9 reference_mode miss-placement

The reference_mode derived syntax is part of the compressed headers and
should have been moved into the corresponding control structure. Document
this mistake. The value can be set to 0 if the driver does not require
compressed headers information.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: fix small typo in comment]