review Java usage of ActorSystem

See https://github.com/apache/pekko/pull/1419#issuecomment-2308390446

terminate() returns a Scala Future

In theory, users can ignore the return value from terminate() and call this:
https://pekko.apache.org/japi/pekko/current/org/apache/pekko/actor/ActorSystem.html#getWhenTerminated()

Maybe, we could also add `terminateWithCompletionStage` that returns `CompletionStage`?

Interestingly, the actor-typed ActorSystem has a `void` return type on `terminate()`.
You have https://pekko.apache.org/japi/pekko/current/org/apache/pekko/actor/typed/ActorSystem.html#getWhenTerminated() for Java users.

Start with the linked pull request comment and compare the classic and typed ActorSystem Java API documentation, especially terminate() and getWhenTerminated(). Determine the intended Java-facing API change and document a concrete decision, including whether a CompletionStage-returning method is required.

Written by the indexing model from the issue text.