Specifies the name of the database to connect to to discover which databases should be vacuumed, when-a/--allis used. If not specified, thepostgresdatabase will be used, or if that does not exist,template1will 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?
It means that in order to query any data from a cluster you must already be connected to a database. Any database. Though to actually vacuum a database you must be connected specifically to it.
I have no clue what you are getting at with your presumption and what specifically you are looking for documentation on. The few places that deal with "whole cluster" type situations, such as here, document the need, like you've quoted here, to connect to a database first before getting said cluster-level information.