doc: Add another 'rule of ownership' to the guide

Inspired by [this question](http://stackoverflow.com/questions/25536654/rust-ownership-rules-in-the-context-of-closures/25537552) on Stack Overflow. I feel like maybe there is an ownership rule missing, in section 18.2, which would describe why you cannot do this:

```
fn main() {
    let mut x = 5i;
    let y = &x; 
    x = 6i; // error: cannot assign to `x` because it is borrowed
}
```

There is an example which shows this error in the closure section, but it actually has nothing to do with closures and is more about a general principle of "the owner can't mutate what it owns while it has been borrowed"
This doesn't quite match the phrasing used by the previous 2 restrictions. Also, I think the restriction list is constructed awkwardly to begin with. Bullet 4 is a restriction just on mutably-lent resources, but bullet 5 and the new bullet 6 are restrictions when the resource is lent both mutable and immutably. I would suggest rephrasing this whole thing as follows:

> Being an owner gives you three privileges:

> 1. You control when that resource is deallocated.
2. You may lend that resource, immutably, to as many borrowers as you'd like.
3. You may lend that resource, mutably, to a single borrower.

> But it also comes with three restrictions:

> 1. You may not lend that resource, mutably, if you're currently lending it to someone (either mutably or immutably).
2. You may not lend that resource, immutably, if you'r currently mutably lending it to someone.
3. You may not mutate that resource if you're currently lending it to someone.