message: Add migration block to tf.compat.v1.Session docstring
diff: @@ -1566,6 +1566,15 @@ class Session(BaseSession):
  allow_soft_placement=True,
  log_device_placement=True))
  ```
  +
  + @compatibility(TF2)
  + `Session` does not work with either eager execution or `tf.function`, and you
  + should not invoke it directly. To migrate code that uses sessions to TF2,
  + rewrite the code without it. See the
  + [migration
  + guide](https://www.tensorflow.org/guide/migrate#1_replace_v1sessionrun_calls)
  + on replacing `Session.run` calls.
  + @end_compatibility
  """
  def __init__(self, target='', graph=None, config=None):
  