message: Update the docstring on function.Defun to reflect current graph/session freezing behavior.
diff: @@ -68,9 +68,10 @@ class Defun(object):
  during the first call to the function. Subsequent function calls will refer to
  the same set of variables.
  - Definitions of functions are frozen in a graph as soon as the graph is used to
  - create a session. Therefore, nodes using the function must be created in the
  - graph before the corresponding session is created.
  + Definitions of functions in a graph are frozen as soon as the graph is used to
  + create a session. However, new functions and new calls to existing functions
  + may be added to the graph, with the new functions themselves becoming
  + immediately frozen.
  Example, but also see the [How To on functions](link_needed).
  