clarifications about foreign_db

Documentation.html

@@ -674,6 +674,9 @@ $cfg['PmaAbsoluteUri'] = (!empty($_SERVER['HTTPS']) ? 'https' : 'http') . '://'
                 click "Structure/Relation view/" and choose foreign fields.
             </li>
         </ul><br />
+        Please note that in the current (2.3.0) version, master_db must be
+        the same as foreign_db.  Those fields have been put in place for a
+        future development of the cross-db relations.<br /><br />
     </dd>
 
     <a name="table_info"></a>