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.
Yeah, adding stuff like `Mutex<T>` will be good in the 'full revamp,' this PR is just to fix things that are actually wrong. I want to wholesale go over this guide in the future, but I figured that removing inaccuracies in the meantime was a good idea.