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_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fd467ceceb0374292bd7384319179c516aecf9b1

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(-)