Google announced smart updates and then enabled it. There is absolutely nothing any developer or user has to do, so there is no documentation on it because none is needed.
OK, that's good news! I stand corrected.
It would still be great for expansion files to get the same treatment, or better yet to abolish expansion files and just allow big APKs.
I'm a little sceptical that absolutely no documentation or feedback is needed. Surely there are things that I as an app developer could be doing to make the deltas smaller.
You complained about managing thin binaries
To clarify, as an Android developer you can manage thin binaries yourself, as a way to reduce the download size; it's just a hassle. My complaint is really that the Play Store should do that app thinning automatically.