Ah, I see that that information is probably retrievable from the source code commit info. It might still be helpful to have a version history for an extension in the docs.
I'm having difficulty finding version difference information about native extensions. For instance, I have an instance running citext version 1.1 and an update command will take it up to 1.6. I was hoping that I'd see some information about version history specific to the extension on the extension's documentation page, but I don't see any reference to extension versioning.