Support accessing __doc__ from cls

[migeed-z]

class Foo:
    @classmethod
    def foo(cls) -> None:
        cls.__doc__

Expectation: no type error
Actual:ERROR 4:9-20: Instance-only attributedocof classFoo is not visible on the class [missing-attribute]

[grievejia]

I suspect this ultimately comes from how type.__doc__ is defined on typeshed. Verified that this PR would fix the issue if it were merged into typeshed. Now we just need to wait and see if typeshed upstream is OK with the patch.

[grievejia]

According to the feedback from upstream, we probably need to relax our check for now and treat every class toplevel attribute as initialized in stub files.