When trying to update a docstring of a CPython builtin, I had problems finding out what Argument Clinic actually does.

First, I looked at the devguide, which does only mention that the clinic exists, but not what it does or how it's used.

Next, I tried "clinic.py --help". This unfortunately doesn't tell anything about itself.

The actual clinic doc is at https://docs.python.org/3/howto/clinic.html

To make the clinic a bit more discoverable, I propose a description including the link to clinic.py (see github #8518).