Clarified assertRaisesMessage() behavior with respect to the message.

docs/topics/testing/tools.txt

@@ -1352,7 +1352,7 @@ your test suite.
             SimpleTestCase.assertRaisesMessage(expected_exception, expected_message)
 
     Asserts that execution of ``callable`` raises ``expected_exception`` and
-    that the exception has an ``expected_message`` representation. Any other
+    that ``expected_message`` is found in the exception's message. 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.