gh-137988: Fix const description in argparse.add_argument() docs

[PrinceNaroliya]

What does this PR do?

This PR fixes a documentation issue in the argparse module related to the add_argument() method when using nargs='?'.

Previously, the docs stated:

"if the option string is encountered with no command-line argument following it, the value of `const` will be assumed to be `None` instead."

This is incorrect. The correct behavior is:

"if the option string is encountered with no command-line argument following it, the value from `const` will be used."

This PR updates the documentation to reflect the correct behavior.

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Related Issue

Fixes: #137988

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Type of Change

• Documentation (typo, clarification, or instruction update)

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How to Verify

1. Open Doc/library/argparse.rst.

2. Locate the passage describing nargs='?' and const.

3. Confirm the updated sentence is:

   "the value from const will be used."

4. Build HTML docs (optional) to verify changes:

   cd Doc
   .\make html
   start _build/html/library/argparse.html

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Checklist

• My change is a documentation fix.
• I have tested the change locally (HTML build).
• I have linked the related issue.

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📚 Documentation preview 📚: https://cpython-previews--138315.org.readthedocs.build/

• Issue: argparse: add_argument(): error about "const" argument #137988

[PrinceNaroliya]

Hi, just checking in on this PR — would really appreciate it if you could take a look whenever you get a chance. Thanks!

[savannahostrowski]

Thank you for the PR! LGTM!

[miss-islington-app]

Thanks @PrinceNaroliya for the PR, and @savannahostrowski for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-139051 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-139052 is a backport of this pull request to the 3.13 branch.