The quickdocs API page on uiop only lists the functions from the package.lisp file. It appears to be related to the fact that uiop defines a macro to define packages and the other package definitions are using that macro, define-package, instead of defpackage.
Yes, there are a lot of other projects that do a better job parsing the API of UIOP. If you don't adopt one of them, one technique you might want to use is detecting the set of packages before and after loading the package and computing the difference.
Also, the project page for uiop is NOT http://common-lisp.net/project/uiop/ which doesn't even exist. Try cliki.net or the asdf page on common-lisp.net.