Fixed #19775 - Clarified requirements of the "default" database.

Databases can have any alias you choose. However, the alias

``default`` has special significance. Django uses the database with

the alias of ``default`` when no other database has been selected. If

you don't have a ``default`` database, you need to be careful to

always specify the database that you want to use.

the concept of a ``default`` database doesn't make sense in the context

of your project, you need to be careful to always specify the database

that you want to use. Django requires that a ``default`` database entry

be defined, but the parameters can be left blank if it will not be used.

The following is an example ``settings.py`` snippet defining two

databases -- a default PostgreSQL database and a MySQL database called