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"