-str #16393 removes timerTransform mentions from scaladocs

This addresses removing references from scaladocs to `timerTransform` that I have missed during this PR: https://github.com/akka/akka/pull/16397

Thanks @patriknw for spotting those, https://github.com/akka/akka/pull/16397#discussion_r20986169
shouldn't this also be `private[akka]` (and also `TransformerLike`)