I tried the third patch and it works, but when I write this into a docstring:

    def func(x, *, y=None):
        """sig=(a,b)"""

then it fails to extract the signature again and returns (a,b) instead.

I also tried putting in some math term as (non-signature) documentation:

    def sig(a, b):
        """sig=(a*b)"""

and it raises a ValueError telling me that the signature is invalid.

So, I still think it should prefer a full function interface over the risky
"__text_signature__" quirk whenever it can.

I also think it shouldn't raise an error if the docstring isn't something
that it can parse, but that's not part of this ticket, I guess.