pointer guide, rc and arc

Adding a little bit of text to the pointer guide on reference counting.

I'd like some more examples, but I'm not sure of good ones.
It's probably worth noting the only way in which `Arc` is more expensive than `Rc` is when actually touching the reference count, i.e. `clone` and when dropping (and, I suppose, when calling `downgrade` or `upgrade`). For general use/`Deref`, Arc is just as cheap as Rc.