message: Document fix to string_view::compare
diff: @@ -48,6 +48,7 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  console output.
  - Fix memory leaks when deserializing URIs and when detaching the content of
  messages (#1160).
  +- Fix undefined behavior in `string_view::compare` (#1164).
  ## [0.18.0-rc.1] - 2020-09-09
  