Add some verbiage on the usage of 'acl_specials' in the default config.inc.php.dist.

plugins/acl/config.inc.php.dist

@@ -16,6 +16,10 @@ $rcmail_config['acl_users_field'] = 'mail';
 // The LDAP search filter will be &'d with search queries
 $rcmail_config['acl_users_filter'] = '';
 
-$rcmail_config['acl_specials'] = array();
+// Include the following 'special' access control subjects in the ACL dialog;
+// Defaults to array('anyone', 'anonymous') (not when set to an empty array)
+// Example: array('anyone') to exclude 'anonymous'.
+// Set to an empty array to exclude all special aci subjects.
+$rcmail_config['acl_specials'] = array('anyone', 'anonymous');
 
 ?>