The documentation for psql saith:
The default user name is your operating-system user name, as is the default database name.
The default database name is the same as the user name chosen for the connection attempt. Thus if one does "postgres~ psql -U davidj" the attempted database name will be davidj, not postgres even though the latter is the operating-system user name in effect.
The documentation for -U doesn't mention this changing of the attempted connect-to database. It is not possible to specify a user name without a database name explicitly given for psql unless one uses -U.
Thus at minimum the above needs to read:
The default user name is your operating-system user name. The default database name is the same as the resolved PostgreSQL user name.
Maybe change the description for -U:
"...of course)." This also changes the default database name.
The parameter dbname in libpq got the memo:
The database name. Defaults to be the same as the user name.
David J.