Document that an existing directory raises in mkdir().

Doc/library/os.rst

@@ -1175,7 +1175,8 @@ Files and Directories
 
    Create a directory named *path* with numeric mode *mode*. The default *mode* is
    ``0777`` (octal).  On some systems, *mode* is ignored.  Where it is used, the
-   current umask value is first masked out.
+   current umask value is first masked out.  If the directory already exists,
+   :exc:`OSError` is raised.
 
    It is also possible to create temporary directories; see the
    :mod:`tempfile` module's :func:`tempfile.mkdtemp` function.