message: Add more doc for the dict returned by an estimator's evaluate() method in the api doc
diff: @@ -431,7 +431,11 @@ class Estimator(object):
  Returns:
  A dict containing the evaluation metrics specified in `model_fn` keyed by
  name, as well as an entry `global_step` which contains the value of the
  - global step for which this evaluation was performed.
  + global step for which this evaluation was performed. For canned
  + estimators, the dict contains the `loss` (mean loss per mini-batch) and
  + the `average_loss` (mean loss per sample). Canned classifiers also return
  + the `accuracy`. Canned regressors also return the `label/mean` and the
  + `prediction/mean`.
  Raises:
  ValueError: If `steps <= 0`.
  