Improve Rc documentation

I've seen quite a bit of confusion around regarding the use of Rc, so I thought I'd take a stab at adding some examples.
This paragraph is a little imprecise in wording. It's not quite true that `Rc` doesn't accept mutable types, as we don't really have a mutable type in the typesystem. You may want to re-word this around these points:

* `Rc` only allows *shared references* (`&T`) to ensure *memory safety*
* *interior mutability* allows mutation of an object through a *shared reference*

We've avoiding saying "immutable by default" because this isn't quite true due to the prevalence of structures like `Cell`, `RefCell`, all concurrent data structures, etc.