message: Improved documentation about eval ops in EstimatorSpec.
diff: @@ -131,7 +131,10 @@ class EstimatorSpec(
  train_op: Op for the training step.
  eval_metric_ops: Dict of metric results keyed by name. The values of the
  dict are the results of calling a metric function, namely a
  - `(metric_tensor, update_op)` tuple.
  + `(metric_tensor, update_op)` tuple. `metric_tensor` should be evaluated
  + without any impact on state (typically is a pure computation results
  + based on variables.). For example, it should not trigger the `update_op`
  + or requires any input fetching.
  export_outputs: Describes the output signatures to be exported to
  `SavedModel` and used during serving.
  A dict `{name: output}` where:
  