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.