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 term "immutable data" isn't quite right as atomics and mutexes are actually how you mutate data (and you put them in arcs). This is largely dated terminology and such.

This section may be a good place to explain things like `AtomicFoo` and `Mutex<T>`?