media: Documentation/media/uapi: more readable unions

Avoid adding an extra columns when describing unions in the documentation.
That makes it much harder to read. See e.g. VIDIOC_QUERY_EXT_CTRLS.

Instead start off a union with 'union {' and end it with an extra row
containing '}'.

This leaves a lot more space for the description of the fields.

This formatting technique was used in a few places already, but this
patches fixes all remaining occurrences of 'union' in the media uAPI.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by: Nicolas Dufresne <nicolas@ndufresne.ca>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>