doc: mention case-insensitive env on windows

On Windows OS, environment variables are case-insensitive and are
treated likewise in NodeJS. This can be confusing and can lead
to hard-to-debug problems when moving code from one environment
to another.

Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/9166
Fixes: https://github.com/nodejs/node/issues/9157