Also, the warning message is a little bit strange: "Non-ascii character appear in docstring"

Suggesting to change the warning message to "Non-ascii characters are not allowed in docstrings: {offending char!r}", and move the check from docstring_for_c_string() to docstring_append(). The former is only called for function docstrings; the latter is called both for function and parameter docstrings.

Linked PRs

• gh-107559: Argument Clinic: complain about non-ascii chars in param docstrings #107560