message: doc: describe native turnserver scope more clearly
  backport of
  Cr-Commit-Position: refs/heads/master@{#30967}
diff: @@ -250,8 +250,7 @@ Utilities for NAT as documented in [RFC 5389][rfc-5389].
  ### TURN Server
  -Target name `turnserver`. In active development to reach compatibility with
  -[RFC 5766][rfc-5766].
  +Target name `turnserver`. Used for unit tests.
  [ninja]: https://ninja-build.org/
  