gh-148542: docs: clarify call by value vs reference in Tutorial 4.8

[dagangtj]

Summary

Clarifies the explanation of argument passing in Tutorial Section 4.8 to reduce confusion about call by value vs reference.

Changes

• Updated the description of how arguments are passed
• Clarified that mutations to mutable objects are visible outside, while reassignments inside the function are not
• Added a short example demonstrating the difference between mutation and reassignment
• Removed the now-redundant footnote

Fixes #148542

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📚 Documentation preview 📚: https://cpython-previews--148563.org.readthedocs.build/