doc: clarify that __dirname is module local

Fixes: https://github.com/nodejs/node/issues/5525
PR-URL: https://github.com/nodejs/node/pull/6018


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>