Re: Patch: Improve Boolean Predicate JSON Path Docs

So, overall reaction to this patch: I like the approach of defining
"predicate check expressions" as being a different thing from standard
jsonpath expressions. However, I'm not so thrilled with just saying
"don't use" one type or the other with different jsonpath functions.
According to my tests, some of these functions seem to give sensible
results anyway with the path type you say not to use, while some
give less-sensible results, and others give errors. We ought to try
to document that, and maybe even clean up the less sane behaviors.
(That is, I don't feel that a docs-only patch is necessarily the
thing to do here.)

As an example, @? seems to behave sanely with a standard jsonpath:

regression=# select '{"a":[1,2,3,4,5]}'::jsonb @? '$.a[*] ? (@ < 5)' ;
?column?
----------
t
(1 row)
regression=# select '{"a":[1,2,3,4,5]}'::jsonb @? '$.a[*] ? (@ > 5)' ;
?column?
----------
f
(1 row)

It will take a predicate, but seems to always return true:

regression=# select '{"a":[1,2,3,4,5]}'::jsonb @? '$.a[*] < 5' ;
?column?
----------
t
(1 row)

regression=# select '{"a":[1,2,3,4,5]}'::jsonb @? '$.a[*] > 5' ;
?column?
----------
t
(1 row)

Surely we're not helping anybody by leaving that behavior in place.
Making it do something useful, throwing an error, or returning NULL
all seem superior to this. I observe that @@ returns NULL for the
path type it doesn't like, so maybe that's what to do here.

(Unsurprisingly, jsonb_path_exists acts similarly.)

BTW, jsonb_path_query_array and jsonb_path_query_first seem to
take both types of path, like jsonb_path_query, so ISTM they need
docs changes too.

regards, tom lane