Adds `noindex` to the robots meta tag.

This directive tells web robots not to index the page content.

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

add_filter( ‘wp_robots’, ‘wp_robots_no_robots’ );

function wp_robots_no_robots( array $robots ) {
	$robots['noindex'] = true;

	if ( get_option( 'blog_public' ) ) {
		$robots['follow'] = true;
	} else {
		$robots['nofollow'] = true;
	}

	return $robots;
}