Documentation for phoenix pubsub websocket protocol

We are looking into developing a C++ library for use with phoenix channels. There are implementations in C#, Swift, Android’s Java, and of course the default/canonical javascript … but we could not find any documentation of the protocol used on the websocket other than the code. There are quite a few (verbose and clear!) comments in the JS implementation that ships with phoenix itself … but it would be fantastic if there was an actual protocol description somewhere.

Does anyone know if such a thing exists? If so, where?

If not, would it be worth anything to others if we wrote one as we write our library?

Thanks in advance!