Add --no-name-check to 'gnt-instance rename' man page

Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

man/gnt-instance.sgml

@@ -1654,6 +1654,7 @@ instance5: 11225
         <cmdsynopsis>
           <command>rename</command>
           <arg>--no-ip-check</arg>
+          <arg>--no-name-check</arg>
           <arg>--submit</arg>
           <arg choice="req"><replaceable>instance</replaceable></arg>
           <arg choice="req"><replaceable>new_name</replaceable></arg>
@@ -1669,6 +1670,14 @@ instance5: 11225
           <option>--no-ip-check</option> option is passed.
         </para>
 
+        <para>
+          The <option>--no-name-check</option> skips the check for the
+          new instance name via the resolver (e.g. in DNS or /etc/hosts,
+          depending on your setup). Since the name check is used to
+          compute the IP address, if you pass this option you must
+          also pass the <option>--no-ip-check</option> option.
+        </para>
+
         <para>
           The <option>--submit</option> option is used to send the job to
           the master daemon but not wait for its completion. The job