#6774: explain shutdown() behavior varying with platform.

Doc/library/socket.rst

@@ -748,7 +748,9 @@ timeout error of its own regardless of any Python socket timeout setting.
    Shut down one or both halves of the connection.  If *how* is :const:`SHUT_RD`,
    further receives are disallowed.  If *how* is :const:`SHUT_WR`, further sends
    are disallowed.  If *how* is :const:`SHUT_RDWR`, further sends and receives are
-   disallowed.
+   disallowed.  Depending on the platform, shutting down one half of the connection
+   can also close the opposite half (e.g. on Mac OS X, ``shutdown(SHUT_WR)`` does
+   not allow further reads on the other end of the connection).
 
 Note that there are no methods :meth:`read` or :meth:`write`; use
 :meth:`~socket.recv` and :meth:`~socket.send` without *flags* argument instead.