Document app.testing=True for test client, #381.

flask/app.py

@@ -716,6 +716,17 @@ class Flask(_PackageBoundObject):
         """Creates a test client for this application.  For information
         about unit testing head over to :ref:`testing`.
 
+        Note that if you are testing for assertions or exceptions in your
+        application code, you must set ``app.testing = True`` in order for the
+        exceptions to propagate to the test client.  Otherwise, the exception
+        will be handled by the application (not visible to the test client) and
+        the only indication of an AssertionError or other exception will be a
+        500 status code response to the test client.  See the :attr:`testing`
+        attribute.  For example::
+
+            app.testing = True
+            client = app.test_client()
+
         The test client can be used in a `with` block to defer the closing down
         of the context until the end of the `with` block.  This is useful if
         you want to access the context locals for testing::