Improve docstrings

tianshou/data/buffer/base.py

@@ -172,18 +172,19 @@ class ReplayBuffer:
         return np.array([last] if not self.done[last] and self._size else [], int)
 
     def prev(self, index: int | np.ndarray) -> np.ndarray:
-        """Return the index of previous transition.
-
-        The index won't be modified if it is the beginning of an episode.
+        """Return the index of preceding step within the same episode if it exists.
+        If it does not exist (because it is the first index within the episode),
+        the index remains unmodified.
         """
-        index = (index - 1) % self._size
+        index = (index - 1) % self._size  # compute preceding index with wrap-around
+        # end_flag will be 1 if the previous index is the last step of an episode or
+        # if it is the very last index of the buffer (wrap-around case), and 0 otherwise
         end_flag = self.done[index] | (index == self.last_index[0])
         return (index + end_flag) % self._size
 
     def next(self, index: int | np.ndarray) -> np.ndarray:
-        """Return the index of next transition.
-
-        The index won't be modified if it is the end of an episode.
+        """Return the index of next step if there is a next step within the episode.
+        If there isn't a next step, the index remains unmodified.
         """
         end_flag = self.done[index] | (index == self.last_index[0])
         return (index + (1 - end_flag)) % self._size