pgsql: doc: spell out full productname

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bdc8d803e8e0eb987b65da1a5528ac570a07bfe1

Modified Files
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doc/src/sgml/pgbuffercache.sgml | 3 ++-
doc/src/sgml/postgres-fdw.sgml  | 3 ++-
2 files changed, 4 insertions(+), 2 deletions(-)