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"
Upon further reflection, even this isn't sufficient. You may also not *read* the resource if it's mutably lent. This includes taking a copy of it, for `Copy` types. The following program is illegal, despite not actually be covered under the rules as listed:

```rust
pub fn main() {
    let mut x = 5i;
    let y = &mut x;
    let z = x;
}
```

Perhaps the following set of restrictions is better:

> 1. If that resource is immutably lent to someone, you may not mutate it or mutably lend it to anyone until the original loan expires.
2. If that resource is mutably lent to someone, you may not access the resource in any fashion until the loan expires.