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_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/480acbbd426863fbaddf0606325e9a49e40bd86c

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(-)