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"
Augh, github commenting race conditions. Was commenting on the first comment, in case it wasn't clear.

I might clarify your new point 2 as:
> If that resource is mutably lent to someone, you may not loan or access the resource in any fashion until the mutable loan expires.