added comment in dstream._test_output

Author: giwa

python/pyspark/streaming/dstream.py

@@ -395,6 +395,10 @@ def _test_output(self, result):
         Store data in a DStream to result to verify the result in test case
         """
         def get_output(rdd, time):
+            """
+            Closure to get element in RDD in the DStream.
+            This closure is called by py4j callback server.
+            """
             collected = rdd.collect()
             result.append(collected)