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.
Notably it may be good to avoid terms like "lightweight" as that tends to invoke the idea of green threading too much (split stacks, etc) which doesn't quite apply to our current task model.