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.

docs/formatdomain.html.in

@@ -1907,7 +1907,12 @@ qemu-kvm -net nic,model=? /dev/null
       <dt><code>video</code></dt>
       <dd>
         The <code>video</code> element is the a container for describing
-        video devices.
+        video devices. For backwards compatability, if no <code>video</code>
+        is set but there is a <code>graphics</code> in domain xml, then libvirt
+        will add a default <code>video</code> according to the guest type.
+        For a guest of type "kvm", the default <code>video</code> for it is:
+        <code>type</code> with value "cirrus", <code>vram</code> with value
+        "9216", and <code>heads</code> with value "1".
       </dd>
 
       <dt><code>model</code></dt>