It is actually 'Module.entity1_entity2'. The function only returns GUID's so it doesn't need to know the entity or attributes.
If you want to use these together with the attribute name on the newly retrieved referenced objects, use a split like this:
var splits = this.nameAttr.split("/");
this.assoc = splits[0]; // 'MyFirstModule.Order_OrderLine'
this.referredEntity = splits[1]; // 'MyFirstModule.OrderLine'
this.myAttribute = splits[2]; // 'DisplayName'
For an example, you can check the Checkbox Selector widget on the AppStore.
I'll update the documentation with the extra information as well.