Add comment that to use LDAP variables in bind_dn/base_dn user_specific must be enabled (#1489335)

config/defaults.inc.php

@@ -646,6 +646,7 @@ $config['ldap_public']['Verisign'] = array(
   'ldap_version'  => 3,       // using LDAPv3
   'network_timeout' => 10,    // The timeout (in seconds) for connect + bind arrempts. This is only supported in PHP >= 5.3.0 with OpenLDAP 2.x
   'user_specific' => false,   // If true the base_dn, bind_dn and bind_pass default to the user's IMAP login.
+  // When 'user_specific' is enabled following variables can be used in base_dn/bind_dn config:
   // %fu - The full username provided, assumes the username is an email
   //       address, uses the username_domain value if not an email address.
   // %u  - The username prior to the '@'.