update class docstring in PiecewiseDecay (#50393)

python/paddle/optimizer/lr.py

@@ -333,7 +333,7 @@ class PiecewiseDecay(LRScheduler):
     Args:
         boundaries(list|tuple): A list/tuple of steps numbers. The type of element in the list is python int.
         values(list|tuple): A list/tuple of learning rate values that will be picked during different epoch boundaries.
-            The type of element in the list is python float.
+            The type of element in the list is python float. The ``values`` have one more element than ``boundaries``.
         last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
         verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .