message: Document `type_id_mapper` addition
diff: @@ -5,6 +5,11 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  ## [Unreleased]
  +### Added
  +
  +- JSON inspectors now allow users to use a custom `type_id_mapper` to generate
  + and parse JSON text that uses different names for the types than the C++ API.
  +
  ### Fixed
  - Passing a response promise to a run-delayed continuation could result in a
  