pgsql: Improve documentation of pageinspect

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

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1fb2d78cb94672f4cfc0c1a2da9e0a534e3698f2

Modified Files
--------------
doc/src/sgml/pageinspect.sgml | 56 +++++++++++++++++++++++++------------------
1 file changed, 33 insertions(+), 23 deletions(-)