bpo-41737: expand doc for NotADirectoryError (GH-27471)

(cherry picked from commit f7c23a99cd4f8179b6ba2cffaeb78b852c0f6488)

Co-authored-by: andrei kulakov <andrei.avk@gmail.com>
diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst
index d5d81df..0e60636 100644
--- a/Doc/library/exceptions.rst
+++ b/Doc/library/exceptions.rst
@@ -690,8 +690,10 @@
 
 .. exception:: NotADirectoryError
 
-   Raised when a directory operation (such as :func:`os.listdir`) is requested
-   on something which is not a directory.
+   Raised when a directory operation (such as :func:`os.listdir`) is requested on
+   something which is not a directory.  On most POSIX platforms, it may also be
+   raised if an operation attempts to open or traverse a non-directory file as if
+   it were a directory.
    Corresponds to :c:data:`errno` ``ENOTDIR``.
 
 .. exception:: PermissionError