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

* 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: