📝 add information about referencing vars from PowerShell to docs

[mason3k]

It is not totally obvious how to reference variables passed to a shell that uses a PowerShell interpreter. The trick is you need to access them as environment variables. I added a brief blurb on how to do this to the docs.

I rebuilt the docs and reviewed my change visually:

The tip at the bottom is what I added.

[nat-n]

Hey @mason3k, thanks for contributing 😄