gh-144837: use standard signature syntax in docs

[jwild2027]

This PR updates documentation method signatures to match standard signature conventions.

Changes:

• Use the positional-only marker , / for extend() where appropriate
• Change pop([i]) to pop(index=-1, /)

Files updated:

• Doc/tutorial/datastructures.rst
• Doc/library/array.rst
• Doc/library/collections.rst

gh-144837

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📚 Documentation preview 📚: https://cpython-previews--144929.org.readthedocs.build/

[python-cla-bot]

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[StanFromIreland]

This is a duplicate of #144841 , please don't open PRs for issues that already have them.