[1.7.x] Fixed #19489 -- Documented host parameter of assertRedirects().

Thanks mrknacky at gmail.com for the report and gajimenezmaggiora
for the draft patch.

Backport of e3c7af18 from master

docs/topics/testing/tools.txt

@@ -1345,16 +1345,24 @@ your test suite.
     You can use this as a context manager in the same way as
     :meth:`~SimpleTestCase.assertTemplateUsed`.
 
-.. method:: SimpleTestCase.assertRedirects(response, expected_url, status_code=302, target_status_code=200, msg_prefix='', fetch_redirect_response=True)
+.. method:: SimpleTestCase.assertRedirects(response, expected_url, status_code=302, target_status_code=200, host=None, msg_prefix='', fetch_redirect_response=True)
 
-    Asserts that the response return a ``status_code`` redirect status, it
-    redirected to ``expected_url`` (including any GET data), and the final
-    page was received with ``target_status_code``.
+    Asserts that the response returned a ``status_code`` redirect status,
+    redirected to ``expected_url`` (including any ``GET`` data), and that the
+    final page was received with ``target_status_code``.
 
     If your request used the ``follow`` argument, the ``expected_url`` and
     ``target_status_code`` will be the url and status code for the final
     point of the redirect chain.
 
+    The ``host`` argument sets a default host if ``expected_url`` doesn't
+    include one (e.g. ``"/bar/"``).  If ``expected_url`` is an absolute URL that
+    includes a host (e.g. ``"http://testhost/bar/"``), the ``host`` parameter
+    will be ignored. Note that the test client doesn't support fetching external
+    URLs, but the parameter may be useful if you are testing with a custom HTTP
+    host (for example, initializing the test client with
+    ``Client(HTTP_HOST="testhost")``.
+
     .. versionadded:: 1.7
 
     If ``fetch_redirect_response`` is ``False``, the final page won't be