Doc Rework: Section 9.16.13 SQL/JSON Query Functions

Hey!

Lots of SQL/JSON threads going about. This one is less about technical
correctness and more about usability of the documentation. Though in
writing this I am finding some things that aren't quite clear. I'm going
to come back with those on a follow-on post once I get a chance to make my
second pass on this. But for the moment just opening it up to a content
and structure review.

Please focus on the text changes. It passes "check-docs" but I still need
to work on layout and stuff in html (markup, some more links).

Thanks!

David J.

p.s. v1 exists here (is just the idea of using basically variable names in
the function signature and minimizing direct syntax in the table);

https://www.postgresql.org/message-id/CAKFQuwbYBvUZasGj_ZnfXhC2kk4AT%3DepwGkNd2%3DRMMVXkfTNMQ%40mail.gmail.com