DOC: add cross reference to -s option to PYTHONNOUSERSITE env

Author: tacaswell

Doc/using/cmdline.rst

@@ -941,8 +941,9 @@ conflict.
 
 .. envvar:: PYTHONNOUSERSITE
 
-   If this is set, Python won't add the :data:`user site-packages directory
-   <site.USER_SITE>` to :data:`sys.path`.
+   This is equivalent to the :option:`-s` option.  If this is set, Python won't
+   add the :data:`user site-packages directory <site.USER_SITE>` to
+   :data:`sys.path`.
 
    .. seealso::