Add a versionadded for PS Core and note that `.venv` is a common virtual environment name.

This is to help prevent people from accidentally installing into the wrong Python interpreter if they are not aware of which Python interpreter `pip` points to.

As part of the update, the documentation was updated to normalize
around the term "virtual environment" instead of relying too heavily
on "venv" for the same meaning and leading to inconsistent usage of
either.

Thanks to Steve Piercy for the patch.