Add doc for extended._retrace_functions_for_each_device.

PiperOrigin-RevId: 313663754
Change-Id: Ie643c0e54adb3eed6a7d20ea3328f2d49fb8047c

tensorflow/python/distribute/distribute_lib.py

@@ -619,6 +619,11 @@ class StrategyBase(object):
 
     if not hasattr(extended, "_retrace_functions_for_each_device"):
       # pylint: disable=protected-access
+      # `extended._retrace_functions_for_each_device` dictates
+      # 1) whether all the ops created inside function will have devices
+      #    inherited from outer stack, and
+      # 2) whether the same function will be retraced when it is called on
+      #    different devices.
       try:
         extended._retrace_functions_for_each_device = (
             len(extended.worker_devices) > 1)