Document more useful model terms

[domenic]

@annevk pointed out that "open" vs. "closed" is nice.

At this point it might be useful to split the model sections into readable and writable streams.

[domenic]

Requested:

"tee", push x to stream, read from stream (and read until empty or read current buffer)
if stream is closed/open I guess

[annevk]

Oh, prepend. See http://encoding.spec.whatwg.org/#concept-stream and maybe read parts of that specification where the concepts are used. Being able to refactor that specification in terms of a new streams model would be indicative of success.