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.

diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 225e0c8cab691d9319c4e8d7546f5f0408b9bda8..98fb2b4de75600a1e66dee2f7060f620e8bf3730 100644 (file)
--- a/docs/formatdomain.html.in
+++ b/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>