doc: proper markdown escaping -> \_\_, \*, \_

PR-URL: https://github.com/nodejs/node/pull/4805


Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>