Update documentation to indicate how PYTHONPATH env variable is used #1328

We've had a number of issues with users complaining about intellisense not working correctly (packages not being located, etc).
The solution to all of this is to update the PYTHONPATH environment variable.
The intellisense engine makes use of PYTHONPATH environment variable.

We may need to surface this in the docs (other IDEs seem to do this).

Brett Cannon (@brettcannon) Mikhail Arkhipov (@MikhailArkhipov)
I understand this is a docs issue, but lets discuss this first.

Going forward this will apply to the new Analysis engine as well.