Add add_policy docstrings

ml-agents/mlagents/trainers/ghost/trainer.py

         return self.trainer.create_policy(brain_parameters)
 
     def add_policy(self, name_behavior_id: str, policy: TFPolicy) -> None:
+        """
+        Adds policy to trainer. For the first policy added, add a trainer
+        to the policy and set the learning behavior name to name_behavior_id.
+        :param name_behavior_id: Behavior ID that the policy should belong to.
+        :param policy: Policy to associate with name_behavior_id.
+        """
         self.policies[name_behavior_id] = policy
         policy.create_tf_graph()
 

ml-agents/mlagents/trainers/ppo/trainer.py

     def add_policy(self, name_behavior_id: str, policy: TFPolicy) -> None:
         """
         Adds policy to trainer.
-        :param brain_parameters: specifications for policy construction
+        :param name_behavior_id: Behavior ID that the policy should belong to.
+        :param policy: Policy to associate with name_behavior_id.
         """
         if self.policy:
             logger.warning(