doc: Fix description of how the default user name is chosen

Hi.

Reposting this on its own thread.

https://www.postgresql.org/message-id/flat/CAKFQuwby1aMsJDMeibaBaohgoaZhivAo4WcqHC1%3D9-GDZ3TSng%40mail.gmail.com

The default database name is just the user name, not the
operating-system user name.

In passing, the authentication error examples use the phrase
"database user name" in a couple of locations. The word
database in both cases is both unusual and unnecessary for
understanding. The reference to user name means the one in/for the
database unless otherwise specified.

Furthermore, it seems better to tell the reader the likely
reason why the displayed database name happens to be a user name.

This change is probably optional but I think it makes sense:
- The indicated database user name was not found.
+ The indicated user name was not found.

The other changes simply try to avoid the issue altogether.

David J.