message: Improve tfdbg documentation regarding high-level APIs
  * Mention both keras and tf.keras
  * In one of the early paragraphs, list all three high-level APIs supported
  (tf.estimator, keras and tf.contrib.slim).
diff: @@ -33,8 +33,9 @@ and [`inf`s](https://en.wikipedia.org/wiki/Infinity), a frequently-encountered
  type of bug in TensorFlow model development.
  The following example is for users who use the low-level
  [`Session`](https://www.tensorflow.org/api_docs/python/tf/Session) API of
  -TensorFlow. A later section of this document describes how to use **tfdbg**
  -with a higher-level API, namely `Estimator`s.
  +TensorFlow. Later sections of this document describe how to use **tfdbg**
  +with higher-level APIs of TensorFlow, including `tf.estimator`,
  +`tf.keras` / `keras` and `tf.contrib.slim`.
  To *observe* such an issue, run the following command without the debugger (the
  source code can be found
  [here](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/debug/examples/debug_mnist.py)):
  @@ -477,20 +478,31 @@ for more details.
  ## Debugging Keras Models with TFDBG
  -To use TFDBG with [Keras](https://keras.io/), let the Keras backend use
  -a TFDBG-wrapped Session object. For example, to use the CLI wrapper:
  +To use TFDBG with
  +[tf.keras](https://www.tensorflow.org/api_docs/python/tf/keras),
  +let the Keras backend use a TFDBG-wrapped Session object. For example, to use
  +the CLI wrapper:
  ``` python
  import tensorflow as tf
  -from keras import backend as keras_backend
  from tensorflow.python import debug as tf_debug
  -keras_backend.set_session(tf_debug.LocalCLIDebugWrapperSession(tf.Session()))
  +tf.keras.backend.set_session(tf_debug.LocalCLIDebugWrapperSession(tf.Session()))
  # Define your keras model, called "model".
  -model.fit(...) # This will break into the TFDBG CLI.
  +
  +# Calls to `fit()`, 'evaluate()` and `predict()` methods will break into the
  +# TFDBG CLI.
  +model.fit(...)
  +model.evaluate(...)
  +model.predict(...)
  ```
  +With minor modification, the preceding code example also works for the
  +[non-TensorFlow version of Keras](https://keras.io/) running against a
  +TensorFlow backend. You just need to replace `tf.keras.backend` with
  +`keras.backend`.
  +
  ## Debugging tf-slim with TFDBG
  TFDBG supports debugging of training and evaluation with
  