doc: spell out full productname
Use PostgreSQL consistently for referring to the productname rather
than Postgres. This also adds <productname> markup.
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: "Jonathan S. Katz" <jkatz@postgresql.org>
Discussion: https://postgr.es/m/
9C019644-9EA4-4B79-A52C-
5C47A5B6B2DF@yesql.se