[Haddock] Documentation for data family instances

[C Rodrigues]

Yes, I agree that instances should be attached to data family declarations.I'll have to puzzle out the instance-collecting code to see how I canextend it to collect data family instances.  I can look into that nextweek.  I'll make this patch independent of the one I wrote before.
> Thanks for the example. You're right that it makes sense to make the
> instances ExportItems.
>
> But I still think the instances should exist in short-form in the
> expItemInstances of the family ExportItem. Do you agree?
>
> Sorry for the late reply!
>
> David