Edits to Guides / Association Basics

This is the Reference section, and I believe I found an error. Or at least, an error by omission. The original text states that `belongs_to` creates a one-to-one relationship. But IMO this is not correct. Taken alone, it creates a **many-to-one** relationship with another model. It is additionally used to create a one-to-one when the other model is given `has_one`. I've updated the guide to state this.