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.
I had a similar comment earlier; steve responded at that time that he's just going with "if it compiles, its fine."  I.e. cross that bridge when we come to it.

So we were waiting until things actually break before putting in the type suffixes or annotations.  We may well have now come to that bridge.