doc: some fixes for environment sections in ref pages

I noticed that the reference pages for initdb and pg_ctl claim in the
Environment section that libpq variables are used, which does not seem
correct to me. I think this was accidentally copied when this blurb was
added to other pages.

While I was checking around that, I also noticed that pg_amcheck and
pg_upgrade don't have Environment sections on their reference pages, so
I added them. For pg_amcheck I copied the standard text for client
programs. pg_upgrade has its own specific list of environment variables.

Patches attached. I think the first one is a bug fix.