message: Update documentation to reflect that XLA_FLAGS and TF_XLA_FLAGS are now split.
diff: @@ -144,7 +144,7 @@ Execute the python script to train the model with XLA and turn on a debugging
  feature of XLA via an environmental variable that outputs the XLA graph.
  ```shell
  -TF_XLA_FLAGS="--xla_hlo_graph_path=/tmp --xla_generate_hlo_graph=.*" python mnist_softmax_xla.py
  +XLA_FLAGS="--xla_hlo_graph_path=/tmp --xla_generate_hlo_graph=.*" python mnist_softmax_xla.py
  ```
  Open the timeline file created (`timeline.ctf.json`). The rendered timeline
  