Adds `noindex` and `noarchive` to the robots meta tag.

This directive tells web robots not to index or archive the page content and
is recommended to be used for sensitive pages.

Typical usage is as a {@see ‘wp_robots’} callback:

add_filter( ‘wp_robots’, ‘wp_robots_sensitive_page’ );

function wp_robots_sensitive_page( array $robots ) {
	$robots['noindex']   = true;
	$robots['noarchive'] = true;
	return $robots;
}