Explain how to properly mock user-defined `__call__`

Bug report

Bug description:

Reproduce

from unittest.mock import MagicMock

class A:
    def __init__(self, a):
        ...

    def __call__(self, b, c):
        ...

mocker = MagicMock(spec=A)
mocker(21, 42)
mocker.assert_called_once_with(21, 42)

Current Behavior

TypeError                                 Traceback (most recent call last)
TypeError: too many positional arguments

The above exception was the direct cause of the following exception:

AssertionError                            Traceback (most recent call last)
Cell In[6], line 10
      8 mocker = MagicMock(spec=A)
      9 mocker(21, 42)
---> 10 mocker.assert_called_once_with(21, 42)

File ~/.pyenv/versions/3.12.2/lib/python3.12/unittest/mock.py:956, in NonCallableMock.assert_called_once_with(self, *args, **kwargs)
    951     msg = ("Expected '%s' to be called once. Called %s times.%s"
    952            % (self._mock_name or 'mock',
    953               self.call_count,
    954               self._calls_repr()))
    955     raise AssertionError(msg)
--> 956 return self.assert_called_with(*args, **kwargs)

File ~/.pyenv/versions/3.12.2/lib/python3.12/unittest/mock.py:944, in NonCallableMock.assert_called_with(self, *args, **kwargs)
    942 if actual != expected:
    943     cause = expected if isinstance(expected, Exception) else None
--> 944     raise AssertionError(_error_message()) from cause

AssertionError: expected call not found.
Expected: mock(21, 42)
  Actual: mock(21, 42)

Expected behaviour

It succeeds

Investigation

When passing a class that implements the __call__ method as spec to a mock instance it takes the it takes the signature of the __init__ which if it is the intended behavior is quite confusing.

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

macOS, Linux

Start with the provided MagicMock reproducer on Python 3.12 or 3.13, then inspect unittest/mock.py around NonCallableMock.assert_called_with and the signature handling involved in spec=A. Compare the behavior for a class defining call with the current traceback. Done means the documented or implemented behavior makes the shown assertion succeed.