message: Add a note on forward compatibility to SavedModelBuilder.save's as_text option.
diff: @@ -395,7 +395,12 @@ class _SavedModelBuilder(object):
  in serialized format.
  Args:
  - as_text: Writes the SavedModel protocol buffer in text format to disk.
  + as_text: Writes the SavedModel protocol buffer in text format to
  + disk. Protocol buffers in text format are useful for debugging, but
  + parsing fails when it encounters an unknown field and so is not forward
  + compatible. This means changes to TensorFlow may prevent deployment of
  + new text format SavedModels to existing serving binaries. Do not deploy
  + `as_text` SavedModels to production.
  Returns:
  The path to which the SavedModel protocol buffer was written.
  