gh-143754: Use neutral terminology in tkinter documentation

[VanshAgarwal24036]

This PR updates the tkinter documentation to replace the term “slave” with
neutral alternatives such as “child widget”.

The changes are limited to documentation wording only and do not affect
APIs or behavior.

Fixes#143754.

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📚 Documentation preview 📚: https://cpython-previews--143779.org.readthedocs.build/

• Issue: tkinter source and docs use the word 'slave' #143754

[VanshAgarwal24036]

This PR is documentation-only, so it does not require a NEWS entry.
Could a maintainer please add the skip news label?

[bedevere-app]

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[VanshAgarwal24036]

Thanks for the clarification.

I’ve updated the paragraph to keep master for the container relationship
while removing slave, and adjusted the wording to stay within the scope
of this PR. I also fixed the winfo_parent markup warning.

Please let me know if this looks good now.

[VanshAgarwal24036]

@terryjreedy I have commited the suggested changes Please review and suggest if any change required

[picnixz]

I think Serhiy's PR addresses this already and in a better way. What has changed between these two PRs? I would also prefer Serhiy's PR as he has deep tkinter knowledge.

[VanshAgarwal24036]

Thanks for the clarification. Since Serhiy’s PR already covers this,
I’m happy to defer. Let me know if you’d like me to close this PR or
if you prefer to handle it.

[terryjreedy]

Thank you for starting this. Serhiy went back to the current tk docs for both docstrings and tkinter.rst.