doc: guidance on SQL/JSON functions vs jsonpath operators

While reading the docs for the SQL/JSON functions added in 17,
I noticed that the current text provides no guidance on when to prefer
these new functions over the existing jsonpath operators.

I was thinking of something like the attached patch,
which introduces a Tip and a Note to clarify the distinction
(focusing on portability/error-handling vs. conciseness/indexing).

PS: Tbh, the documentation around these PG17 additions is currently quite
dense. It likely needs more comprehensive content - which I may provide
soon - but this seemed like an obvious first step.