fix obsolete references to postgres.h in comments

Nathan Bossart <nathandbossart@gmail.com>

From: Nathan Bossart <nathandbossart@gmail.com>
To: pgsql-hackers@postgresql.org
Date: 2025-09-18T19:20:29Z
Lists: pgsql-hackers

Attachments

Most of these were missed by commit d952373a98, but I found one that seems
to have been missed by commit d08741eab5.

-- 
nathan

Commits

  1. Fix obsolete references to postgres.h in comments.

  2. New header varatt.h split off from postgres.h

  3. Restructure the key include files per recent pghackers discussion: there