docs: add missing docstring for Broker.join() function (Bogdanp#579)

dramatiq/broker.py

@@ -286,7 +286,16 @@ def flush_all(self): # pragma: no cover
  raise NotImplementedError()
 
  def join(self, queue_name, *, timeout=None): # pragma: no cover
- """
+ """Wait for all the messages on the given queue to be
+ processed. This method is only meant to be used in tests to
+ wait for all the messages in a queue to be processed.
+
+ Subclasses that implement this function may add parameters.
+
+ Parameters:
+ queue_name(str): The queue to wait on.
+ timeout(Optional[int]): The max amount of time, in
+ milliseconds, to wait on this queue.
  """
  raise NotImplementedError()