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.
We generally recommend instead of making the entire object inside of a `RefCell` that rather each individual field is in a `Cell` or `RefCell`. This allows finer-grained access and may prevent borrow violations.

In this example, I think it may work better if the `gadgets` field of `Owner` were in a `RefCell`. This also ends up expressing the invariant that `gadgets` may change, but `name` never will.