How would you expect to extract this docstring?

I'm not sure how this would work in practice, since both of these are errors:

>>> class A:
...    def __init__(self):
...        self.x = 3
...        self.x.__doc__ = 'foo'
...
>>> A()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __init__
AttributeError: 'int' object attribute '__doc__' is read-only
>>> class B:
...    x: int = 0
...    x.__doc__ = 'foo'
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in B
AttributeError: 'int' object attribute '__doc__' is read-only

It could be stored in the dataclass-specific data attached to a class, but then you'd have to use a dataclass-specific function to get access to it. I'm not sure that's a great improvement.

I also note that attrs doesn't have this feature, probably for the same reason.