docs: domaincaps: Mention VIR_DOMAIN_UNDEFINE_CHECKPOINTS_METADATA

The flag for the virDomainUndefine API is supported even if we report
that <backup supported='no'/>. Mention it in the docs.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>

docs/formatdomaincaps.html.in

@@ -566,7 +566,10 @@
 
     <p>Reports whether the hypervisor supports the backup, checkpoint, and
     related features. (<code>virDomainBackupBegin</code>,
-    <code>virDomainCheckpointCreateXML</code> etc).
+    <code>virDomainCheckpointCreateXML</code> etc). The presence of the
+    <code>backup</code> element even if <code>supported='no'</code> implies that
+    the <code>VIR_DOMAIN_UNDEFINE_CHECKPOINTS_METADATA</code> flag for
+    <code>virDomainUndefine</code> is supported.
     </p>
 
     <h4><a id="elementsSEV">SEV capabilities</a></h4>