gh-143754: Modernize Tkinter docs

[serhiy-storchaka]

Use more up to date terminology instead of "master"/"slave" widgets where it is possible.

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

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📚 Documentation preview 📚: https://cpython-previews--143841.org.readthedocs.build/

[serhiy-storchaka]

I used the modern Tk documentation to update the Tkinter documentation.