message: Document `message_builder` fix
diff: @@ -29,6 +29,8 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  newer EC-curve API.
  - When working with settings, `put`, `put_missing`, `get_if`, etc. now
  gracefully handle the `global` category when explicitly using it.
  +- Messages created from a `message_builder` did not call the destructors for
  + their values, potentially causing memory leaks (#1321).
  ### Changed
  