doc: add more details to process.env

process.env has a few quirks that deserve documenting.

The commit documents:

- How assigning to process.env will implicitly call `toString()`
- How to remove an environment variable from process.env

PR-URL: https://github.com/nodejs/node/pull/4924


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>