pgsql: Document handling of invalid/ambiguous timestamp input nearDST

Document handling of invalid/ambiguous timestamp input near DST boundaries.

The source code comments documented this, but the user-facing docs, not
so much.  Add a section to Appendix B that discusses it.

In passing, improve a couple other things in Appendix B --- notably,
a long-obsolete claim that time zone abbreviations are looked up in
a fixed table.

Per bug #15527 from Michael Davidson.

Discussion: https://postgr.es/m/15527-f1be0b4dc99ebbe7@postgresql.org

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6247c0bc2ded1fc92edc55341d9544573e862531

Modified Files
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doc/src/sgml/datetime.sgml | 90 ++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 84 insertions(+), 6 deletions(-)