Adds `noindex` to the robots meta tag if a search is being performed.

If a search is being performed then noindex will be output to
tell web robots not to index the page content. Add this to the
{@see ‘wp_robots’} filter.

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

add_filter( ‘wp_robots’, ‘wp_robots_noindex_search’ );

function wp_robots_noindex_search( array $robots ) {
	if ( is_search() ) {
		return wp_robots_no_robots( $robots );
	}

	return $robots;
}