bpo-33722: Add documentation to unittest.mock.mock_open

Doc/library/unittest.mock.rst

@@ -2213,6 +2213,16 @@ And for reading files::
     >>> m.assert_called_once_with('foo')
     >>> assert result == 'bibble'
 
+If you want to mock open for use in another module you can mock
+builtins.open:
+
+    >>> with patch('builtins.open', mock_open(read_data='bibble')) as m:
+    ...     with open('foo') as h:
+    ...         result = h.read()
+    ...
+    >>> m.assert_called_once_with('foo')
+    >>> assert result == 'bibble'
+
 
 .. _auto-speccing:
 

Misc/NEWS.d/next/Documentation/2019-04-10-14-47-17.bpo-33722.UwwygU.rst

@@ -0,0 +1,2 @@
+Add in example to unittest.mock.rst showing how to mock builtins.open in
+cases where builtins.open is mocked in another module.