pgsql: Doc: clarify behavior of timestamptz input some more.

Doc: clarify behavior of timestamptz input some more.

Try to make it absolutely plain that we don't retain the
originally specified time zone, only the UTC timestamp.

While at it, make glossary entries for "UTC" and "GMT".

Author: Robert Treat <rob@xzilla.net>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/173796426022.1064.9135167366862649513@wrigleys.postgresql.org
Backpatch-through: 13

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fe47f3897e99ec07ddc4ab4fcd777e546c8cc030

Modified Files
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doc/src/sgml/datatype.sgml | 17 ++++++++++-------
doc/src/sgml/glossary.sgml | 16 ++++++++++++++++
2 files changed, 26 insertions(+), 7 deletions(-)