Describe each contrib module in its SGML section title

Describe each contrib module in its SGML section title

The original titles only had the module name, which is not very useful
when scanning the list.  By adding a very brief description to each
title, the table of contents becomes friendlier.

Also amend the introduction in the "additional modules" appendix, using
the word "Extension" more extensively.  Nowadays, almost all contrib
modules are extensions, so this is also helpful.

Author: Karl O. Pinc <kop@karlpinc.com>
Reviewed-by: Brar Piening <brar@gmx.de>
Discussion: https://postgr.es/m/20230102180015.372995a9@slate.karlpinc.com