Fix lies in the concurrency guide.

This cleans up blatant lies in the concurrency guide, and modernizes it
a bit. There's a lot more to do, but until I get to it, let's make it a
little bit better.
The "come in a variety of forms" is actually no longer true as well (we only have one `Sender`/`Receiver`). Although I suppose `SyncSender` is another kind...