ODC: add note to PYTHONUSERBASE about PYHTONNOUSERSITE

Otherwise it is not clear how to disable user site-packages.  Even though it is
directly above, the link from the search results put the top of the page on
PYHONUSERBASE and the user may not think to scroll up for critical context.

Author: tacaswell

Doc/using/cmdline.rst

@@ -957,6 +957,9 @@ conflict.
    and :ref:`installation paths <sysconfig-user-scheme>` for
    ``python -m pip install --user``.
 
+   To disable the user site-packages, see :envar:`PYTHONNOUSERSITE` or the :option:`-s`
+   option.
+
    .. seealso::
 
       :pep:`370` -- Per user site-packages directory