docs: Update doc on how to mock HTTP responses.

We prefer to use respones module rather than a
custom system to mock the HTTP responses.

Rewritten by tabbott to provide more context/explanation.

docs/testing/testing-with-django.md

@@ -301,9 +301,15 @@ from Django as well as our own custom helpers.  Here is an example:
 Follow [this link](../subsystems/settings.html#testing-non-default-settings) for more
 information on the "settings" context manager.
 
-A common use is to prevent a call to a third-party service from using
-the Internet; `git grep mock.patch | grep requests` is a good way to
-find several examples of doing this.
+Zulip has several features, like outgoing webhooks or social
+authentication, that made outgoing HTTP requests to external
+servers. We test those features using the excellent
+[responses](https://pypi.org/project/responses/) library, which has a
+nice interface for mocking `requests` calls to return whatever HTTP
+response from the exteranl server we need for the test. you can find
+examples with `git grep responses.add`. Zulip's own `HostRequestMock`
+class should be used only for low-level tests for code that expects to
+receive Django HttpRequest object.
 
 ## Zulip testing philosophy