Incomplete example in documentation

The first example in the documentation (`friends-of`) leave the `friends-of` implementation empty, but it also doesn't mention anywhere what kind of object it should return.

In the end I found out it just has to be a value wrapped in `urania.core/value` but I guess it would be nice to have a fully working example in the docs to try out.
I can make a PR if that's useful I made up a working example.