I don't think it was ever a requirement of docstrings that their signature fit on one line or that they render fully in IDLE.  Other built-in functions have multi-line signatures going back 10+ years (e.g. 32e7d0898eab).

I still think that the rendering of multi-line signatures should be considered separately.  The discussion and issues are different, and people may have different opinions.  For example, why not be smarter about detecting the end of the signature (e.g. first line not having "->")?  Would you object to creating a new issue?