Hi Tim,
For IFLOW & BPM, as per my understanding the documentation is not available as it is a pictorial representation & we can get an overview of the interface looking through the representation itself.
For ESR and ID objects we have description which can be added in Swing Tool and NWDS both the places.
Adding the screenshot from NWDS, swing tool you already stated it is available.
Also, for ESR objects in NWDS we have documentation link which we can edit/view for interfaces.
Thanks,
Nidhi Srivastava