In Doc/howto/descriptor.rst:

    # Internally, the bound method stores the underlying function,
    # the bound instance, and the class of the bound instance.
    >>> d.f.__func__
    <function D.f at 0x1012e5ae8>
    >>> d.f.__self__
    <__main__.D object at 0x1012e1f98>
    >>> d.f.__class__
    <class 'method'>

The bound method (PyMethodObject) does not store "the class of the bound instance" - it only stores the "function" and "self".
d.f.__class__ is the class of the "method" type itself, not the class of d.f's instance (D from d = D())

I think this mention should be removed from the documentation?

Good catch.  Thanks for the report.

The origin of the error was an incorrect update from Python 2 semantics where the class was exposed as an attribute:

Python 2.7.17 (v2.7.17:c2f86d86e6, Oct 19 2019, 16:24:34) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license()" for more information.
>>> class A:
	def f(self, x):
		return x
	
>>> a = A()
>>> bm = a.f
>>> bm.im_class
<class __main__.A at 0x1002daf30>
>>> bm.im_self
<__main__.A instance at 0x103a204b0>
>>> bm.im_func
<function f at 0x1004ed950>

Thanks for the PR.