Good point - we discussed this at the developer hangout and there isn’t a great universal solution given how many libraries slicer builds on an integrates. Here’s a very old page that lists and links back to the various pieces.
We also noticed that while Qt’s Python documentation is good, it’s not perfect either. For example this python documentation just lists the method type, while the corresponding C++ documentation includes useful text.
It would be great if someone wants to build a better doc infrastructure, but for now I suggest looking at the source code. And of course you can use help in the python interactor.
Hope that helps