gh-89819: Add argument_default and conflict_handler to add_argument_group docs

[savannahostrowski]

• Issue: argparse bug: prefix_chars argument to add_argument_group doesn't really work #89819

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📚 Documentation preview 📚: https://cpython-previews--125379.org.readthedocs.build/

[savannahostrowski]

@serhiy-storchaka I think this covers the bases described in the issue, but if you have something more substantial in mind, please let me know.

[serhiy-storchaka]

It is a little more complex.

• These parameters are keyword-only. Add the * marker before them.
• None is not their default value. Actually, their default values cannot be expressed in a syntax of Python function declaration, because they are current values of the parser. We can use brackets, like in add_argument() and add_subparsers().
• These parameters should be at least barely mentioned in the following description. Not necessary to add a bullet list for all parameters. But there should be references to corresponding descriptions of the parser parameters, with possible description of the differences (or say that they are the same). This is the only non-trivial part.

[serhiy-storchaka]

LGTM.

[miss-islington-app]

Thanks @savannahostrowski for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12, 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-125538 is a backport of this pull request to the 3.13 branch.

[bedevere-app]

GH-125539 is a backport of this pull request to the 3.12 branch.