add rpc documentation

This PR aims to add documentation for the RPC code, and make it more clear what calls are available, and how they work. This should also make it easier in the future to write clients in other languages.

I'm not sure if this initial state is good enough, please collaborate and give feedback where needed :)

I am not sure if I should omit the (incomplete) type info or not, for now it's there, but if you rather have me remote that part, I'll do so.