[2.1.x] Refs #29513 -- Linked the testing docs from the multi-db topic guide.

Backport of 2a011626 from master

docs/topics/db/multi-db.txt

@@ -7,6 +7,11 @@ multiple databases. Most of the rest of Django's documentation assumes
 you are interacting with a single database. If you want to interact
 with multiple databases, you'll need to take some additional steps.
 
+.. seealso::
+
+    See :ref:`testing-multi-db` for information about testing with multiple
+    databases.
+
 Defining your databases
 =======================
 

docs/topics/testing/tools.txt

@@ -1107,6 +1107,8 @@ tests can't rely upon the fact that your views will be available at a
 particular URL. Decorate your test class or test method with
 ``@override_settings(ROOT_URLCONF=...)`` for URLconf configuration.
 
+.. _testing-multi-db:
+
 Multi-database support
 ----------------------