doc: minor improvement in OS docs

Add links to `process.arch` and `process.platform`.

PR-URL: https://github.com/nodejs/node/pull/5006


Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>