I'd like to propose the following slight rewording for the note about ALTER TABLE ... ADD table_constraint_using_index. The existing text says one may "install it [the existing index] as an official constraint".
The "official constraint" phrase sounds like weasel wording to me -- how is a UNIQUE constraint more "official" than an UNIQUE index? Then "install" makes it sound like some installation process is going on that you should be aware of. I like "convert" slightly better instead.
Josh