DOC: document include-system-site-packages interaction with usersite

It is not otherwise documented that this implicitly disables user
site-packages.

Author: tacaswell

Doc/library/site.rst

@@ -64,7 +64,8 @@ When running under a :ref:`virtual environment <sys-path-init-virtual-environmen
 the ``pyvenv.cfg`` file in :data:`sys.prefix` is checked for site-specific
 configurations. If the ``include-system-site-packages`` key exists and is set to
 ``true`` (case-insensitive), the system-level prefixes will be searched for
-site-packages, otherwise they won't.
+site-packages, otherwise they won't.  If the system-level prefixes are not searched then
+the user site prefixes are also implicitly not searched for site-packages.
 
 .. index::
    single: # (hash); comment

Lib/site.py

@@ -20,7 +20,9 @@
 installation). If "pyvenv.cfg" (a bootstrap configuration file) contains
 the key "include-system-site-packages" is set to  "true"
 (case-insensitive), the system-level prefixes will still also be
-searched for site-packages; otherwise they won't.
+searched for site-packages; otherwise they won't.  If the system-level
+prefixes are not included then the user site prefixes are also implicitly
+not searched for site-packages.
 
 All of the resulting site-specific directories, if they exist, are
 appended to sys.path, and also inspected for path configuration