Improve documentation of pageinspect
This adds a section for heap-related functions. These were previously
mixed with functions having a more general purpose, leading to
confusion. While on it, add a query example for fsm_page_contents.
Backpatch down to 10, where
b5e3942 introduced the subsections for
function types in pageinspect documentation.
Author: Masahiko Sawada
Discussion: https://postgr.es/m/CAD21AoDyM7E1+cK3-aWejxKTGC-wVVP2B+RnJhN6inXyeRmqzw@mail.gmail.com
Backpatch-through: 10