Document relationship custom attributes on bulk contact updates (Preview) (#793)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: VarshSuresh

descriptions/0/api.intercom.io.yaml

@@ -42408,7 +42408,9 @@ components:
                 example: fr
               custom_attributes:
                 type: object
-                description: Custom attributes to update on the contact.
+                description: Custom attributes to update on the contact. For a
+                  relationship attribute, send the IDs of the custom object records
+                  to link the contact to.
               companies:
                 type: array
                 description: A list of companies to associate with the contact.