Explainer should explain the API design choices

[domenic]

E.g.:

• Why use streams? Especially for UDP sockets? Consider using streams for UDP sockets as well #17 is relevant. (To be clear, I think using streams is the right decision!)

• Explain the connection-establishment style. Probably after doing Align with WebSocketStream on opening #18 which suggests a better design :).

• Comparison to other popular socket APIs, e.g. B2G, Chrome Apps, Node.js.