pgsql: Doc: improve documentation for jsonpath behavior.

Doc: improve documentation for jsonpath behavior.

Clarify the behavior of jsonpath operators and functions by
describing their two different modes of operation explicitly.
In addition to the SQL-spec behavior, where a path returns
a list of matching items, we have a "predicate check" form
that always returns a single boolean result. That was mentioned
in only one place, but it seems better to annotate each operator
and function as to which form(s) it takes. Also improve the
examples by converting them into actual executable SQL with
results, and do a bunch of incidental wordsmithing.

David Wheeler, reviewed by Erik Wienhold, Jian He, and myself

Discussion: https://postgr.es/m/7262A188-59CA-4A8A-AAD7-83D4FF0B9758@justatheory.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7014c9a4bba2d1b67d60687afb5b2091c1d07f73

Modified Files
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doc/src/sgml/func.sgml | 389 ++++++++++++++++++++++++++++++++++---------------
doc/src/sgml/json.sgml | 2 +-
2 files changed, 271 insertions(+), 120 deletions(-)