Fixed #21222 - Documented that 'default' and 'other' must be different databases…

Fixed #21222 - Documented that 'default' and 'other' must be different databases when running tests.

Thanks vernondcole for the suggestion.

docs/internals/contributing/writing-code/unit-tests.txt

@@ -64,11 +64,11 @@ two databases:
 * A ``default`` database. This database should use the backend that
   you want to use for primary testing
 
-* A database with the alias ``other``. The ``other`` database is
-  used to establish that queries can be directed to different
-  databases. As a result, this database can use any backend you
-  want. It doesn't need to use the same backend as the ``default``
-  database (although it can use the same backend if you want to).
+* A database with the alias ``other``. The ``other`` database is used to
+  establish that queries can be directed to different databases. As a result,
+  this database can use any backend you want. It doesn't need to use the same
+  backend as the ``default`` database (although it can use the same backend if
+  you want to). It cannot be the same database as the ``default``.
 
 If you're using a backend that isn't SQLite, you will need to provide other
 details for each database: