message: Add reference from unstable doc anchor to stable env documentation
diff: @@ -1413,3 +1413,10 @@ The `extra-link-arg` feature to specify additional linker arguments in build
  scripts has been stabilized in the 1.56 release. See the [build script
  documentation](build-scripts.md#outputs-of-the-build-script) for more
  information on specifying extra linker arguments.
  +
  +### configurable-env
  +
  +The `configurable-env` feature to specify environment variables in Cargo
  +configuration has been stabilized in the 1.56 release. See the [config
  +documentation](config.html#env) for more information about configuring
  +environment variables.
  