message: Update ECRelationship inheritance rules docs
  After adding tests and discussing with Colin, we decided to update the inheritance rules and delete two which we don't enforce anyways, and which in reality are not followed.
diff: @@ -17,7 +17,6 @@ Relationship class rules:
  Relationship inheritance rules:
  - Relationships only support single inheritance
  -- Relationships may only derive from an abstract relationship
  - Derived relationships narrow relationship constraints of their parent relationship
  - Constraint classes must be equal to or more derived than those defined in a base class
  - Multiplicities must be equal to or more restrictive than those defined in a base class
  @@ -28,7 +27,6 @@ Relationship inheritance rules:
  - Role labels may be customized with no restrictions in derived relationships.
  - An abstract constraint class defined in a derived relationship must be or derive from the base abstract constraint class
  -- Only base relationships may define properties
  - Only base relationships may be referenced by a Navigation property
  - Only base relationships may set the order by attribute
  