bpo-43917: Fix pure python equivalent for classmethod (GH-25544)

Reported by Yahor Harunovich.

Doc/howto/descriptor.rst

@@ -1329,7 +1329,7 @@ Using the non-data descriptor protocol, a pure Python version of
         def __get__(self, obj, cls=None):
             if cls is None:
                 cls = type(obj)
-            if hasattr(obj, '__get__'):
+            if hasattr(type(self.f), '__get__'):
                 return self.f.__get__(cls)
             return MethodType(self.f, cls)
 
@@ -1342,6 +1342,12 @@ Using the non-data descriptor protocol, a pure Python version of
         def cm(cls, x, y):
             return (cls, x, y)
 
+        @ClassMethod
+        @property
+        def __doc__(cls):
+            return f'A doc for {cls.__name__!r}'
+
+
 .. doctest::
     :hide:
 
@@ -1353,6 +1359,11 @@ Using the non-data descriptor protocol, a pure Python version of
     >>> t.cm(11, 22)
     (<class 'T'>, 11, 22)
 
+    # Check the alternate path for chained descriptors
+    >>> T.__doc__
+    "A doc for 'T'"
+
+
 The code path for ``hasattr(obj, '__get__')`` was added in Python 3.9 and
 makes it possible for :func:`classmethod` to support chained decorators.
 For example, a classmethod and property could be chained together: