media: doc: codec: Rename "Return fields" to "Returned fields" for consistency

The parts about fields that are returned typically comes after
"Required fields" so it feels more consistent to name them
"Returned fields".

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>