doc: make os api doc more consistent

This adds a missing Returns to os.arch() as well as a missing added in
version to os.constants.

PR-URL: https://github.com/nodejs/node/pull/10994


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>