WordPress/wp-admin/options-head.php

<?php
/**
 * WordPress Options Header.
 *
 * Displays updated message, if updated variable is part of the URL query.
 *
 * @package WordPress
 * @subpackage Administration
 */

$action = ! empty( $_REQUEST['action'] ) ? sanitize_text_field( $_REQUEST['action'] ) : '';

if ( isset( $_GET['updated'] ) && isset( $_GET['page'] ) ) {
	// For back-compat with plugins that don't use the Settings API and just set updated=1 in the redirect.
	add_settings_error( 'general', 'settings_updated', __( 'Settings saved.' ), 'success' );
}

settings_errors();