media: vidioc-enum-fmt.rst: make the ENUM_FMT text clearer

Rework the documentation to make it easier for the reader to understand
the differences in behavior of this ioctl between MC and non-MC drivers.

Note the addition of the 'video-node-centric' and 'MC-centric' terms to
help understand what the IO_MC capability really means.

Also mention in the beginning that mbus_code is one of the fields that
application should initialize, and add META_OUTPUT as one of the types that
this ioctl supports (that was never added here when the META_OUTPUT buffer
type was added).

Finally document that EINVAL will be returned if mbus_code is unsupported.

Fixes: e5b6b07a ("media: v4l2: Extend VIDIOC_ENUM_FMT to support MC-centric devices")
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>