I think we can now agree that docstrings other than the class docstring (used as a fallback) are not relevant to signature detection. And Raymond gave namedtuple classes the docstring needed as a fallback.

We are off-issue here, but idlelib.CallTips.get_argspec() is also ignorant that it may need to look at  .__new__. An object with a C-coded .__init__ and Python-coded .__new__ is new to new-style classes. The new inspect.signature function handles such properly. Starting with a namedtuple Point (without the default docstring):

>>> from inspect import signature
>>> str(signature(Point.__new__))
'(_cls, x, y)'
>>> str(signature(Point))
'(x, y)'

The second is what autodoc should use. I just opened #19903 to update Idle to use signature.