Hi Stuart, FYI and for the records, the documentation is actually embedded in the SDK download, in the zedi editor.
Yes, it took me a while to find out, and I'm not entirely sure it is not embedded somewhere else, but at least it is here if you need it.
I'm on a mac, so I just run it with java -jar zexplore.jar in the terminal and dismiss the warning it gived, I'm sure the .exe on windows is easier.
Once loaded, you choose the SDK version in the top left drop down, like "Ontapi 1.21 Cluster Mode", and you get a nice hierarchy of calls with attributes and return values.