message: Document recently deprecated APIs
diff: @@ -43,6 +43,16 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  - The types `caf::byte`, `caf::optional` and `caf::string_view` became obsolete
  after switching to C++17. Consequently, these types are now deprecated in
  favor of their standard library counterpart.
  +- The group-based pub/sub mechanism never fit nicely into the typed messaging
  + API and the fact that group messages use the regular mailbox makes it hard to
  + separate regular communication from multi-cast messages. Hence, we decided to
  + drop the group API and instead focus on the new flows and streams that can
  + replace group-communication in many use cases.
  +- The "actor-composition operator" was added as a means to enable the first
  + experimental streaming API. With that gone, there's no justification to keep
  + this feature. While it has some neat niche-applications, the prevent some
  + optimization we would like to apply to the messaging layer. Hence, we will
  + remove this feature without a replacement.
  ### Removed
  