message: Document API changes
diff: @@ -42,6 +42,14 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  losing connection to the monitored node.
  - In preparation of potential future API additions/changes, CAF now includes an
  RFC4122-compliant `uuid` class.
  +- The new trait class `is_error_code_enum` allows users to enable conversion of
  + custom error code enums to `error` and `error_code`.
  +
  +### Deprecated
  +
  +- The `to_string` output for `error` now renders the error code enum by default.
  + This renders the member functions `actor_system::render` and
  + `actor_system_config::render` obsolete.
  ### Changed
  