message: Add a little docstring
diff: @@ -519,6 +519,8 @@ pub fn path_source(source_id: SourceId, config: &Config) -> CargoResult<PathSour
  Ok(PathSource::new(&path, source_id, config))
  }
  +/// Either the specific dependency to be selected, or a function which can be executed to list all
  +/// packages known in a source so that one can be selected from that list.
  pub enum DependencyOrListAllFn<F> {
  Dependency(Dependency),
  ListAllFn(F),
  