> What would be really slick would be to have a tool that looks at the
> functions in pg_proc, and generates a DocBook document that lists all
> of the functions, their arguments (at least at the type level), and
> the comment attached to each.

Might want to take a look at PostgreSQL Autodoc.

www.rbt.ca/autodoc/

It does 90% of this already. It has an option to output docbook XML,
which works great with xmlinclude's to suck in content segments if you
don't want all of the data.