bpo-38623: Add note about site module (site-packages) (GH-16974) (GH-28536)

(cherry picked from commit 55b45bf)

Co-authored-by: Peter Bittner <[email protected]>

Author: miss-islington

Doc/tutorial/modules.rst

@@ -191,7 +191,8 @@ named :file:`spam.py` in a list of directories given by the variable
   file is specified).
 * :envvar:`PYTHONPATH` (a list of directory names, with the same syntax as the
   shell variable :envvar:`PATH`).
-* The installation-dependent default.
+* The installation-dependent default (by convention including a
+  ``site-packages`` directory, handled by the :mod:`site` module).
 
 .. note::
    On file systems which support symlinks, the directory containing the input