Update docstrings

rasa/core/tracker_store.py

@@ -351,14 +351,24 @@ def save(self, tracker, timeout=None):
         )
 
     def retrieve(self, sender_id: Text) -> Optional[DialogueStateTracker]:
+        """Retrieves tracker for the latest conversation session.
+
+        The Redis key is formed by appending a prefix to sender_id.
+
+        Args:
+            sender_id: Conversation ID to fetch the tracker for.
+
+        Returns:
+            Tracker containing events from the latest conversation sessions.
+        """
         stored = self.red.get(self.key_prefix + sender_id)
         if stored is not None:
             return self.deserialise_tracker(sender_id, stored)
         else:
             return None
 
     def keys(self) -> Iterable[Text]:
-        """Returns keys of the Redis Tracker Store"""
+        """Returns keys of the Redis Tracker Store."""
         return self.red.keys(self.key_prefix + "*")