Add useful/helpful comment to retry scheduler

This is hopefully useful to understand why this happens
in this order.

Change-Id: I8ff63417489a8d4bada12acc91dbc6bca69df2ba

taskflow/engines/action_engine/scheduler.py

@@ -42,6 +42,9 @@ class RetryScheduler(object):
             return self._retry_action.schedule_reversion(retry)
         elif intention == st.RETRY:
             self._retry_action.change_state(retry, st.RETRYING)
+            # This will force the subflow to start processing right *after*
+            # this retry atom executes (since they will be blocked on their
+            # predecessor getting out of the RETRYING/RUNNING state).
             self._runtime.retry_subflow(retry)
             return self._retry_action.schedule_execution(retry)
         else: