message: Clarify that _as_graph_element methods may not add to the graph.
diff: @@ -3458,7 +3458,8 @@ class Graph(object):
  Args:
  obj: A `Tensor`, an `Operation`, or the name of a tensor or operation. Can
  also be any object with an `_as_graph_element()` method that returns a
  - value of one of these types.
  + value of one of these types. Note: `_as_graph_element` will be called
  + inside the graph's lock and so may not modify the graph.
  allow_tensor: If true, `obj` may refer to a `Tensor`.
  allow_operation: If true, `obj` may refer to an `Operation`.
  