Add comment for save_flag trait.

IPython/core/history.py

@@ -80,6 +80,7 b' class HistoryManager(Configurable):'
     
     # History saving in separate thread
     save_thread = Instance('IPython.core.history.HistorySavingThread')
+    # N.B. Event is a function returning an instance of _Event.
     save_flag = Instance(threading._Event)
     
     # Private interface