gh-142093: clarify which variables appear in function.__closure__

[Vemulakonda559]

This PR adds a clarification to the documentation of function.__closure__,
noting that only variables from an enclosing scope that are actually
referenced in the inner function are included in its closure.

Names defined in the outer function but not referenced inside the nested
function do not appear in co_freevars and therefore do not contribute
cells to function.__closure__.

This behavior follows from the definition of free variables in the
execution model, but placing this clarification directly in the
function.__closure__ section improves interpretability for readers
examining closure behavior.

Documentation-only change; no behavioral modifications.

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📚 Documentation preview 📚: https://cpython-previews--142086.org.readthedocs.build/

• Issue: Clarify which variables appear in function.__closure__ #142093

[python-cla-bot]

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[picnixz]

Ok, after searching a bit more, we could just link the glossary term: "closure variable" which includes this clarification.

[picnixz]

Sorry for telling you to create an issue. I thought it needed more discussion but I think we can just add a reference to the closure variable or free variable term instead.