docs: Add doc for video element
docs: Add doc for video element

For backwards compatibility, if no <video> is set but there is a
<graphics> tag, then we add a default <video> according to the
guest type. Add docs to tell the user about this to not make
them confused. Especially if they remove the video (such as via
"virsh edit"), it will be surprised for them to see the video
element is still in domain XML.

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