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.
C has `NULL` macro, C++ has also `nullptr`. But is it even necessary to mention about `null` here? I would rather expect better explanation what `()` is and what it has to do with semicolons. The fact that `()` is both name of a type and the only value of that type might be confusing for someone who doesn't know rust so it definitively needs good explanation.