Rework SQL/JSON documentation

This makes the documentation conform more closely the style of other
function sections.

Includes suggestions from Erik Rijkers, Alvaro Herrera, and Justin
Pryzby.

Discussion: https://postgr.es/m/CAKFQuwaq5Ny6d3nWbJo=QO4RmhSk9JD8zrkURLR-wWmB2Pkz7Q@mail.gmail.com

Author: adunstan