Adds `rel=”nofollow”` string to all HTML A elements in content.

function wp_rel_nofollow( $text ) {
	// This is a pre-save filter, so text is already escaped.
	$text = stripslashes( $text );
	$text = preg_replace_callback(
		'||i',
		static function( $matches ) {
			return wp_rel_callback( $matches, 'nofollow' );
		},
		$text
	);
	return wp_slash( $text );
}