Glossary and initdb definition work for "superuser" and database/cluster

Hey,

Recent threads have pointed out some long-standing doc language in initdb
that could be made more precise, especially in light of the relatively
recent addition of a glossary. Toward this end I'm attaching a patch that
defines three terms: "bootstrap superuser", "database superuser" and
"superuser". I didn't add any extra-glossary links for the later two but
did for the limited-in-scope bootstrap superuser that is really only
defined in initdb (actually, I suspect the authorization docs could use a
link too but haven't gone looking for an appropriate place yet).

In passing I also changed a few places where the documentation says
"database" when the thing being referred to is basically the file system
data directory, which is a cluster-scoped thing.

I did some grep'ing, though another pass or two is probably worthwhile.
For now I submit a preliminary patch for consideration and buy-in before
trying to polish it up.

David J.