Guide: if

Adding the section about if to the guide. I hope this won't conflict with https://github.com/rust-lang/rust/pull/15229 , but we'll just have to see, I guess.
This seems to imply that this line sets both `y` and `x` to `5i`, whereas in reality (because assignments return `()`) it sets `y` to `()` and `x` to `5i`.