Add "how to" for the setter Argument Clinic directive

[corona10]

See: python/cpython#112922

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📚 Documentation preview 📚: https://cpython-devguide--1245.org.readthedocs.build/

[erlend-aasland]

Hm, instead of having two how-to's -- one for @getter and one for @setter -- how about unifying them and creating a "How to declare PyGetSetDef functions"? After all, Argument Clinic's audience is (mainly) developers who are fairly comfortable with the C API.

[AlexWaygood]

(I'm travelling right now, but will do my best to review this tomorrow or Wednesday!)

[erlend-aasland]

I left some suggestions.

[AlexWaygood]

Thanks! Some minor points:

[AlexWaygood]

Not sure this should be deleted; the Python REPL example at the bottom now has no introduction explaining what it's illustrating

[corona10]

Okay, I will supplement to the above descriptions.

[corona10]

Hmm.. I think that "Getters" and "setters" are C functions defined in a :c:type:`PyGetSetDef` struct that facilitate :py:class:`property`-like access for a class will be enough for understand Python REPL..

[AlexWaygood]

Thanks!