Clarify range's exclusivity.

Inspired by http://www.reddit.com/r/rust/comments/298js2/what_is_the_rationale_behind_the_second_parameter/
What about an example that shows you exactly what the range is?
```rust
let array = [0, 1, 2, 3, 4];
    
for i in range(0, 5u) {
    assert_eq!(i,  array[i]);
}
```
Another way could be to sum the range, but that seems more confusing
```rust
let mut accumulator = 0;
for i in range(0, 5u) {
    accumulator += i;
}

println!("The sum of range(0, 5u) is 0+1+2+3+4 = {}", accumulator);
```