Document how to use varargs in Argument Clinic

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This is related to https://github.com/python/cpython/issues/64490

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

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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 https://github.com/python/cpython/issues/64490, **kwargs is not supported by Argument Clinic.