Remove incorrect documentation

[shawwn]

The documentation of mtf.while_loop implies that control_dependencies doesn't work on TPUs. This was true early in the lifecycle of TPU development, but TPUs respect control_dependencies now. I emailed TFRC to ask about this. It was a similar situation: someone on our team found documentation in a tensorflow codebase somewhere that implied TPUs don't respect control_dependencies. So now I'm going around attempting to remove such inaccuracies, as it's no longer relevant.

It's possible that this documentation might be correct if it's referring to some special XLA op rather than tf.control_dependencies, but it sounds like it's talking about tf.control_dependencies.