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.
Ah. We are talking about two different things. I am talking about `let` here, which is a statement, not an expression. You are talking about assignment, which is an expression. In expression-based languages, they are the same.

With that in mind, let me try clarifying this...