Document methods that return None

If a valid broker in the cluster has no partitions, it will return None rather than an empty set.

Similarly updated a few other methods.

Author: jeffwidman

kafka/cluster.py

@@ -96,6 +96,7 @@ def available_partitions_for_topic(self, topic):
 
         Returns:
             set: {partition (int), ...}
+            None if topic not found.
         """
         if topic not in self._partitions:
             return None
@@ -119,6 +120,7 @@ def partitions_for_broker(self, broker_id):
 
         Returns:
             set: {TopicPartition, ...}
+            None if the broker either has no partitions or does not exist.
         """
         return self._broker_partitions.get(broker_id)
 
@@ -130,6 +132,7 @@ def coordinator_for_group(self, group):
 
         Returns:
             int: node_id for group coordinator
+            None if the group does not exist.
         """
         return self._groups.get(group)