Fixed #20166 -- Updated DB dependency documentation and fixed example

Updated the documentation for how to control the order in which
tests databases are created by making it clear that the reason the
default is always created first is because by default all databases
are made dependent on the default one. Also fixed the example
to be correct as before this change it was actually raising
the ImproperlyConfigured exception

docs/topics/testing/advanced.txt

@@ -113,7 +113,8 @@ two databases.
 Controlling creation order for test databases
 ---------------------------------------------
 
-By default, Django will always create the ``default`` database first.
+By default, Django will assume all databases depend on the ``default``
+database and therefore always create the ``default`` database first.
 However, no guarantees are made on the creation order of any other
 databases in your test setup.
 
@@ -129,6 +130,7 @@ can specify the dependencies that exist using the
         },
         'diamonds': {
             # ... db settings
+             'TEST_DEPENDENCIES': []
         },
         'clubs': {
             # ... db settings