message: Document recent changes to the response promises
diff: @@ -22,6 +22,8 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  trying to parse the input of `x` if it contains a string. The function
  `get_or` already existed for `settings`, but we have added new overloads for
  generalizing the function to `config_value` as well.
  +- The `typed_response_promise` received additional member functions to mirror
  + the interface of the untyped `response_promise`.
  ### Deprecated
  @@ -84,6 +86,8 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  - Skipping high-priority messages resulted in CAF lowering the priority to
  normal. This unintentional demotion has been fixed (#1171).
  - Fix undefined behavior in the experimental datagram brokers (#1174).
  +- Response promises no longer send empty messages in response to asynchronous
  + messages.
  ## [0.18.0-rc.1] - 2020-09-09
  