Clarify prose

This change clearly explains the role that the PATH environment variable
has in creating virtual environments using virtualenv, and contrasts
that behaviour with that of venv (which only uses the Python it was run
with).

Author: zahlman

source/key_projects.rst

@@ -338,9 +338,10 @@ virtualenv
 `GitHub <https://github.com/pypa/virtualenv>`__ |
 `PyPI <https://pypi.org/project/virtualenv/>`__
 
-virtualenv is a tool which uses the command-line path environment
-variable to create isolated Python :term:`Virtual Environments
-<Virtual Environment>`, much as :ref:`venv` does. virtualenv provides
+virtualenv is a tool for creating isolated Python :term:`Virtual Environments
+<Virtual Environment>`, like :ref:`venv`. Unlike :ref:`venv`, virtualenv can
+create virtual environments for other versions of Python, which it locates
+using the PATH environment variable. It also provides
 additional functionality, compared to :ref:`venv`, by supporting Python
 2.7 and by providing convenient features for configuring, maintaining,
 duplicating, and troubleshooting the virtual environments. For more