vacuumdb maintenance-db??

Specifies the name of the database to connect to to discover which databases should be vacuumed, when -a/--all is used. If not specified, the postgres database will be used, or if that does not exist, template1 will be used.
Presumably this means that I can create a tiny database with one table that lists a set of databases. Or maybe not.

Where is this documented?

Thanks