update docstring and add note about differences in behaviour

Author: jorisvandenbossche

pandas/core/strings/accessor.py

@@ -3610,10 +3610,19 @@ def casefold(self):
     >>> s3 = pd.Series(['23', '³', '⅕', ''])
     >>> s3.str.isdigit()
     0     True
-    1    False
-    2    False
+    1     True
+    2     True
     3    False
     dtype: bool
+
+    Notes
+    -----
+    The exact behavior of this method, i.e. which unicode characters are
+    considered as digits, depends on the backend used for string operations,
+    and there can be small differences.
+    For example, Python considers the ³ superscript character as a digit, but
+    not the ⅕ fraction character, while PyArrow considers both as digits. For
+    simple (ascii) decimal numbers, the behaviour is consistent.
     """
 
     _shared_docs["isspace"] = """

pandas/tests/strings/test_strings.py

@@ -7,8 +7,10 @@
 import numpy as np
 import pytest
 
-from pandas.compat import pa_version_under21p0
-from pandas.errors import Pandas4Warning
+from pandas.compat import (
+    Pandas4Warning,
+    pa_version_under21p0,
+)
 
 from pandas import (
     NA,