doc: add process api data types to documentation

PR-URL: https://github.com/nodejs/node/pull/9505


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Roman Reiss <me@silverwind.io>