docs(api): add a short note about the `target_node` argument

api/libs/broadcast_channel/redis/sharded_channel.py

@@ -70,6 +70,10 @@ class _RedisShardedSubscription(RedisSubscriptionBase):
         # Since we have already filtered at the caller's site, we can safely set
         # `ignore_subscribe_messages=False`.
         if isinstance(self._client, RedisCluster):
+            # NOTE(QuantumGhost): due to an issue in upstream code, calling `get_sharded_message`
+            # would use busy-looping to wait for incoming message, consuming excessive CPU quota.
+            #
+            # Here we specify the `target_node` to mitigate this problem.
             node = self._client.get_node_from_key(self._topic)
             return self._pubsub.get_sharded_message(
                 ignore_subscribe_messages=False,