Add docstring to public method on AxClient (#1755)

Summary:
Pull Request resolved: #1755

As titled

Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: saitcakmak

Differential Revision: D47720983

fbshipit-source-id: fe4ca6b7b46a0681d3a5d84825207101fe2350ba

ax/service/ax_client.py

@@ -873,6 +873,12 @@ def get_trial_parameters(self, trial_index: int) -> TParameterization:
         return not_none(self.get_trial(trial_index).arm).parameters
 
     def get_trials_data_frame(self) -> pd.DataFrame:
+        """Get a Pandas DataFrame representation of this experiment. The columns
+        will include all the parameters in the search space and all the metrics
+        on this experiment. The rows will each correspond to a trial (if using
+        one-arm trials, which is the case in base ``AxClient``; will correspond
+        to arms in trials in the batch-trial case).
+        """
         return exp_to_df(exp=self.experiment)
 
     def get_max_parallelism(self) -> List[Tuple[int, int]]: