Guide: functions

Just a few words about functions and defining them.
Please remove the final ", we do not believe it to be possible" from the sentence.

Text like "we do not believe it to be possible" is inviting derisive retorts from PL nerds, uh, I mean, "experts" who will point out cases much like the one that huon points out, and variants which can be more subtle, such as this one:
```rust
fn foo(flag: bool) {
    let z = if flag {
        2
    } else {
        3
    } // <-- this is the spot where one might or might not have
      // intended a semicolon.

    -bar(); // (its not illegal to negate and then discard a result; 
            // for some types the negate may even have side-effects)
    println!("is z: {} == 2?", z);
}

fn bar() -> int { 5 }

fn main() {
    foo(true)
}
```