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

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

This makes the distinction between operating-system user name and
database user name a bit clearer.  It also clarifies that the user
name is determined first, and then the default database name.

Author: David G. Johnston <david.g.johnston@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CAKFQuwZUhgz=sUi+wGQV-PBTNjMovuA-BOV88RV-Vw0m0drCAg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/385da7306c7fce210bd059ad97ebb3e19699e161

Modified Files
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doc/src/sgml/client-auth.sgml  |  2 +-
doc/src/sgml/ref/psql-ref.sgml | 15 ++++++++-------
2 files changed, 9 insertions(+), 8 deletions(-)