Update documentation.

    // responsible for refreshing the alerts should be loaded in the page.

    $this->assertJavaScriptPresent();

    // Refresh the page for a few seconds until the alert appears. Thanks to the

    // Check that the alert appears within a few seconds. Thanks to the

    // workaround this will work regardless of the fact that the page is cached.

    $this->assertSiteAlertAppears('Scheduled alert');