message: Document streaming fix for the CAF 0.17 series
diff: @@ -223,6 +223,8 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  parent broker on creation, which prevented proper system shutdown in some
  cases. Adding all servants consistently to the broker should make sure UDP
  brokers terminate correctly (#1133).
  +- Backport stream manager fix from CAF 0.18 for fused downstream managers that
  + prevent loss of messages during regular actor shutdown.
  ## [0.17.6] - 2020-07-24
  