Document how to use varargs in Argument Clinic

[aisk]

This is related to python/cpython#64490

---

📚 Documentation preview 📚: https://cpython-devguide--1277.org.readthedocs.build/

[ezio-melotti]

This might be higher up in the list.
Should **kwargs be documented too, possibly together with *args?

[aisk]

Thanks for the review!

This might be higher up in the list.

Is it ok to move it below How to convert METH_O and METH_NOARGS functions?

Should **kwargs be documented too, possibly together with *args?

According to python/cpython#64490, **kwargs is not supported by Argument Clinic.