Mention array sugar in guide
auto merge of #17535 : Manishearth/rust/patch-2, r=steveklabnik

It's a rather useful syntax, and non-obvious.

A friend of mine is learning Rust and was trying to find a way to easily do such an initialization — he couldn't find it in the guide and was pretty surprised when I showed him. Looks like something that should be mentioned.

r? @steveklabnik