add comment about AmbiguousTimeError

Author: jackzyliu

pandas/tests/tseries/offsets/test_offsets_properties.py

@@ -101,8 +101,8 @@ def test_on_offset_implementations(dt, offset):
     try:
         compare = (dt + offset) - offset
     except (pytz.NonExistentTimeError, pytz.AmbiguousTimeError):
-        # dt + offset does not exist, assume(False) to indicate
-        #  to hypothesis that this is not a valid test case
+        # When dt + offset does not exist or is DST-ambiguous, assume(False) to
+        # indicate to hypothesis that this is not a valid test case
         assume(False)
 
     assert offset.is_on_offset(dt) == (compare == dt)