Document that IntoIterator allows for for...in sugar

[frewsxcv]

http://doc.rust-lang.org/core/iter/trait.IntoIterator.html

I think we should mention somewhere in the docs for IntoIterator that implementing this trait allows one to write:

let my_vec = vec![1, 10, 100];
for i in my_vec { ... }

instead of

let my_vec = vec![1, 10, 100];
for i in my_vec.iter() { ... }

[FreeFull]

What about an explanation of for x in &values and for x in &mut values?

[frewsxcv]

That seems more of a conceptual, in-depth question that should probably be answered in the Book rather than API documentation

[Gankra]

@FreeFull Or would be good in the top-level Iterator or Collections docs.