Complete --node description for gnt-backup(8)

The second paramenter of --node was not described in gnt-backup. Add
information about it.

Reviewed-by: imsnah

man/gnt-backup.sgml

@@ -205,6 +205,11 @@
         </variablelist>
       </para>
 
+      <para>
+        The optional second value of the <option>--node</option> is used for
+        the remote raid template type and specifies the remote node.
+      </para>
+
       <para>
         If you do not want gnt-backup to wait for the disk mirror
         to be synced, use the <option>--no-wait-for-sync</option>