What I just tried locally is to add a package-protected Java `interface SnapshotStorePlugin` in `akka.persistence.snapshot.japi`, with  all the javadocs that are currently in `akka-docs`. I also removed the API docs from `akka.persistence.snapshot.japi.SnapshotStore`.

- with `genjavadoc:doc` the API docs are copied from interface `SnapshotStorePlugin` to `SnapshotStore`
- with `doc`, the Java API docs are not copied, which is acceptable IMO as Java users will probably use the generated javadocs and Scala users can still read the Scala API docs in the `akka.persistence.snapshot.SnapshotStore` super class .
- in both cases `SnapshotStorePlugin` doesn't show up in the API docs but we can include it in the generated user docs for Java.

I'll follow the same approach for the journal plugin APIs as well. If you don't mind I'll include these changes with my next pull request which I hope to have ready later this week.