Add warning concerning bookmark feature and advanced auth

Documentation.html

@@ -345,6 +345,11 @@
             </li>
         </ul>
         <br />
+	For reasons of security, if you want to use bookmark feature and
+	advenced authentification, you must configure a different cfgServers
+	section for each user.
+	<br />
+	<br />
     </dd>
 
     <dt><b>$cfgServerDefault</b> integer</dt>