docs: Describe new 'append' attribute for chardevs source

Signed-off-by: NDmitry Mishin <dim@virtuozzo.com>

docs/formatdomain.html.in

@@ -5227,7 +5227,7 @@ qemu-kvm -net nic,model=? /dev/null
       <target port='0'/>
     </serial>
     <serial type='file'>
-      <source path='/tmp/file'>
+      <source path='/tmp/file' append='on'>
         <seclabel model='dac' relabel='no'/>
       </source>
       <target port='0'/>
@@ -5263,6 +5263,14 @@ qemu-kvm -net nic,model=? /dev/null
         the per-domain setting</a>.
     </p>
 
+    <p>
+      If the interface <code>type</code> presented to the host is "file",
+      then the <code>source</code> element may contain an optional attribute
+      <code>append</code> that specifies whether or not the information in
+      the file should be preserved on domain restart. Allowed values are
+      "on" and "off" (default). <span class="since">Since 1.3.1</span>.
+    </p>
+
     <p>
       Each character device element has an optional
       sub-element <code>&lt;address&gt;</code> which can tie the