Update REST API docs about Relationship Type Resource

This resource will be required for People Management task.

List/search, add, edit, retire, and delete Relationship Types. Relationship Types define the types of relationships between Persons. Also, manage how relationship types are shown to the user (which types are shown by default and in which order).

Relationships have two directions. Some relationships differ depending on the direction; for example, if A is related to B as "Parent", then B is, by definition, related to A as "Child". Other relationships are the same in either direction (e.g., the reverse relationship of "Sibling" is also "Sibling").

Edit RelationShipTypeResource1_8 implementation if applicable.