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.
(or we could just delete the sentence and the example.  I worry that this portion of the text is targeting at one kind of programmer, coming from a Ruby-like background, but will be terribly confusing to someone coming from other backgrounds, where trying to explain what is going on in the Ruby example will require more background text than we really want to provide here.)