Documentation of `Lwt_stream.create` and `Lwt_stream.get`

[ezal]

It seems that pushing None to a stream closes the stream. I think this not said, and it could be said in the doc-string for create which creates the push function with type 'a option -> unit and it is not clear why option is used.

Related to this, the documentation for get is:

(** [get st] removes and returns the first element of the stream, if
    any. *)

This implies that if the stream is empty, then None is returned immediately (as a fulfilled promise).

But, AFAIU, that's not the semantics of get. If the stream is empty, get "blocks" until either a value v is available, in which case Some v is "returned". And get "returns" None if the stream is closed.

I can do a PR if I'm not wrong about the above.