message: Document ordering constraint on providing thread tokens
diff: @@ -96,6 +96,9 @@ pub struct JobQueue<'a, 'cfg> {
  tokens: Vec<Acquired>,
  rustc_tokens: HashMap<JobId, Vec<Acquired>>,
  +
  + // We use a vec here as we don't want to order randomly which rustc we give
  + // tokens to.
  to_send_clients: Vec<(JobId, Client)>,
  pending_queue: Vec<(Unit<'a>, Job)>,
  print: DiagnosticPrinter<'cfg>,
  