Added docs for assertRaisesMessage as context manager.

docs/topics/testing/tools.txt

@@ -1284,13 +1284,21 @@ message generated by the assertion. This allows you to provide additional
 details that may help you to identify the location and cause of an failure in
 your test suite.
 
-.. method:: SimpleTestCase.assertRaisesMessage(expected_exception, expected_message, callable_obj=None, *args, **kwargs)
+.. method:: SimpleTestCase.assertRaisesMessage(expected_exception, expected_message, callable, *args, **kwargs)
+            SimpleTestCase.assertRaisesMessage(expected_exception, expected_message)
 
-    Asserts that execution of callable ``callable_obj`` raised the
-    ``expected_exception`` exception and that such exception has an
-    ``expected_message`` representation. Any other outcome is reported as a
-    failure. Similar to unittest's :meth:`~unittest.TestCase.assertRaisesRegex`
-    with the difference that ``expected_message`` isn't a regular expression.
+    Asserts that execution of ``callable`` raises ``expected_exception`` and
+    that the exception has an ``expected_message`` representation. Any other
+    outcome is reported as a failure. Similar to unittest's
+    :meth:`~unittest.TestCase.assertRaisesRegex` with the difference that
+    ``expected_message`` isn't a regular expression.
+
+    If only the ``expected_exception`` and ``expected_message`` parameters are
+    given, returns a context manager so that the code being tested can be
+    written inline rather than as a function::
+
+        with self.assertRaisesMessage(ValueError, 'invalid literal for int()'):
+            int('a')
 
 .. method:: SimpleTestCase.assertFieldOutput(fieldclass, valid, invalid, field_args=None, field_kwargs=None, empty_value='')