module documentation

"Rel objects" for related fields. "Rel objects" (for lack of a better name) carry information about the relation modeled by a related field and provide some utility functions. They're stored in the ``remote_field`` attribute of the field. They also act as reverse fields for the purposes of the Meta API because they're the closest concept currently available.

Class ForeignObjectRel Used by ForeignObject to store information about the relation.
Class ManyToManyRel Used by ManyToManyField to store information about the relation.
Class ManyToOneRel Used by the ForeignKey field to store information about the relation.
Class OneToOneRel Used by OneToOneField to store information about the relation.