add documentation for sleep's "ref" argument

gevent/hub.py

@@ -85,6 +85,9 @@ def sleep(seconds=0, ref=True):
     If *seconds* is equal to or less than zero, yield control the other coroutines
     without actually putting the process to sleep. The :class:`core.idle` watcher
     with the highest priority is used to achieve that.
+
+    If *ref* is false, the greenlet running sleep() will not prevent gevent.run()
+    from exiting.
     """
     hub = get_hub()
     loop = hub.loop