Improve documentation and testing of jsonpath string() for datetimes.

Improve documentation and testing of jsonpath string() for datetimes.

Point out that the output format depends on DateStyle, and test that,
along with testing some cases previously not covered.

In passing, adjust the horology test to verify that the prevailing
DateStyle is 'Postgres, MDY', much as it has long verified the
prevailing TimeZone.  We expect pg_regress to have set these up,
and there are multiple regression tests relying on these settings.

Also make the formatting of entries in table 9.50 more consistent.

David Wheeler (marginal additional hacking by me); review by jian he

Discussion: https://postgr.es/m/56955B33-6959-4FDA-A459-F00363ECDFEE@justatheory.com