The scope of this project is to define what a Zope interface is, how documentation for those interfaces should be written, and to write said documentation.
There are many, many examples out there of successful API documentation. One particular example we can take a lot of direct experience from is the Python Library Reference. In particular, we don't necessarily need to re-invent any wheels to get very far along on this project.
Third party developers need to be able to provide interface documentation for their products that can, at the least, fit into the presentation of "offical" APIs with minimal effort without tying them to specific policies.