Guide: closures

For the next closure revision:
> Also, I believe it's idiomatic for short simple one-expression closures to just be written without the { ... }, e.g. |x: int| x * x - by @huonw 

Kimundi posted [here](http://www.reddit.com/r/rust/comments/2e0jn9/rustdev_byvalue_variable_captures_landing_soon/cjvktbx) an explanation of the updated closures. This example from his work looks sorta like mixing of those types:
```rust
let mut counter = 0u;
let a = 128;

let counter_ref = &mut counter;
let by_mix = |b| if b > a { *counter_ref += 1; }; 
// Captures a and counter_ref as value

by_mix(64);
by_mix(256);
by_mix(1024);
assert!(counter == 2)
```

Specifically here it looks like a mixing of the *no braces allowance* and requiring it. It also looks a little like a [pattern guard](http://doc.rust-lang.org/tutorial.html#pattern-matching) on a match:
```rust
let by_mix = |b| if b > a { *counter_ref += 1; }; 
```

I don't know if it's worth calling out an example like this in the future or not but I wanted to point it out in case it's worth it.