pgsql: Doc: document EXTRACT(JULIAN ...), improve Julian Date explanati

Doc: document EXTRACT(JULIAN ...), improve Julian Date explanation.

For some reason, the "julian" option for extract()/date_part() has
never gotten listed in the manual.  Also, while Appendix B mentioned
in passing that we don't conform to the usual astronomical definition
that a Julian date starts at noon UTC, it was kind of vague about what
we do instead.  Clarify that, and add an example showing how to get
the astronomical definition if you want it.

It's been like this for ages, so back-patch to all supported branches.

Discussion: https://postgr.es/m/1197050.1619123213@sss.pgh.pa.us

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b230618ce8759147e838c3a08709685cddca2468

Modified Files
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doc/src/sgml/datetime.sgml | 46 ++++++++++++++++++++++++++++++++++++++++------
doc/src/sgml/func.sgml     | 22 +++++++++++++++++++++-
2 files changed, 61 insertions(+), 7 deletions(-)