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

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