doc: improve docstring for tuner.best_estimator()

Author: knakad

src/sagemaker/tuner.py

@@ -760,6 +760,19 @@ def best_estimator(self, best_training_job=None):
         be deployed to an Amazon SageMaker endpoint and return a ``sagemaker.RealTimePredictor``
         object.
 
+        Args:
+            best_training_job (dict): dict: Dictionary containing "TrainingJobName" and
+                "TrainingJobDefinitionName".
+                Example:
+                {
+                "TrainingJobName": "my_training_job_name",
+                "TrainingJobDefinitionName": "my_training_job_definition_name"
+                }
+
+        Returns:
+            sagemaker.estimator.EstimatorBase: The estimator that has the best training job
+                attached.
+
         Raises:
             Exception: If there is no best training job available for the hyperparameter tuning job.
         """