Add warning about PYTHONPATH

See conversation here:

#136880

for the request for this documentation update.

Author: glennglazer

Doc/tutorial/venv.rst

@@ -88,6 +88,8 @@ For example:
   '~/envs/tutorial-env/lib/python3.5/site-packages']
   >>>
 
+Note that the activated virtual environment manages the :term:`PYTHONHOME` environment variable to find the specific version of Python that is requested. It does not alter the :term:`PYTHONPATH` variable in any way. This may lead to unexpected results if the path includes references to code which is incompatible with the Python version the virtual environment is using. The best practice is to ``unset PYTHONPATH`` in bash or the equivalent for the shell you are using.
+
 To deactivate a virtual environment, type::
 
     deactivate