message: Document bugfix on the actor registry cleanup
diff: @@ -11,6 +11,9 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  encountering `SSL_ERROR_WANT_READ`. The crash manifested if CAF resumed a
  write operation but failed to fully reset its state. The state management (and
  consequently the crash) has been fixed.
  +- CAF now clears the actor registry before calling the destructors of loaded
  + modules. This fixes undefined behavior that could occur in rare cases where
  + actor cleanup code could run after loaded modules had been destroyed.
  ## [0.18.4] - 2021-07-07
  