DOC: fix docstring of timedaltas.ceil (#60047)

* fix docstring of timedaltas.cela

* Update pandas/_libs/tslibs/timedeltas.pyx

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

---------

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

Author: eightyseven

pandas/_libs/tslibs/timedeltas.pyx

@@ -2178,8 +2178,10 @@ class Timedelta(_Timedelta):
         Parameters
         ----------
         freq : str
-            Frequency string indicating the ceiling resolution.
-            It uses the same units as class constructor :class:`~pandas.Timedelta`.
+            Frequency string indicating the ceiling resolution. Must be a fixed
+            frequency like 's' (second) not 'ME' (month end). See
+            :ref:`frequency aliases <timeseries.offset_aliases>` for
+            a list of possible `freq` values.
 
         Returns
         -------