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.
Indeed, clarifying that was part of the intent of my feedback here: https://github.com/rust-lang/rust/pull/15276#discussion_r14454120  but I have at this point assumed that @steveklabnik is deliberately trying to avoid discussing this distinction at this point in the tutorial.