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