I want to autogenerate some documentation for some cross-language bindings I'm making, and so it would be nice to have access to the documentation in raw form.

I've had related problems to this, in my case it was because I was using an old version of macOS and Xcode, that didn't have the newer Apple Worldwide Developer Relations certificate installed. You can install it yourself by downloading it from https://www.apple.com/certificateauthority/.