bpo-26330: Update shutil.disk_usage() documentation (pythonGH-5184)

Clarify that on Windows, path must be a directory.
On Unix, path can be a file or a directory.
(cherry picked from commit ee3b835)

Author: csabella

Doc/library/shutil.rst

@@ -318,7 +318,8 @@ Directory and files operations
 
    Return disk usage statistics about the given path as a :term:`named tuple`
    with the attributes *total*, *used* and *free*, which are the amount of
-   total, used and free space, in bytes.
+   total, used and free space, in bytes.  On Windows, *path* must be a
+   directory; on Unix, it can be a file or directory.
 
    .. versionadded:: 3.3