doc: add `added:` information for util

PR-URL: https://github.com/nodejs/node/pull/8206


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>