documentation: media: vivid: Update documentation on vivid loopback support

Modify section "Video and Sliced VBI Looping" in Documentation to explain
the vivid loopback support for video across multiple vivid instances.
Previous documentation is out-of-date as it was explaining looping in a
single vivid instance only.

Also, in "Some Future Improvements" the item "Add support to loop
from a specific output to a specific input across vivid instances"
can be dropped since that's now implemented.

Signed-off-by: Dorcas Anono Litunya <anonolitunya@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>