Correcting / giving full details for belongs_to.

The original text reads that belongs_to creates a one-to-one relationship. But I don't believe this is correct. As the diagram shows, it actually creates a many-to-one relationship.  Additionally, belongs_to can be used to create one side of a one-to-one relationship, with the other model using has_one. But on its own, it sets up many-to-one.