doc: add details about rss on process.memoryUsage

1. `process.memoryUsage()` returns an object with 4 keys: `rss,
heapTotal, headUsed, external`. There were brief explanations for
the rest except `rss`. This commit adds this on the docs.

2. A little more clarity on `rss` to help people disambiguate it from
the virtual memory size.

PR-URL: https://github.com/nodejs/node/pull/16566
Refs: https://github.com/nodejs/node/pull/16566#discussion_r147545405


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>