gh-133525: doc that PYTHONEXECUTABLE is not Mac-only and respected with -E/-I options

[rickeylev]

This corrects the docs about how PYTHONEXECUTABLE is handled, specifically:

• It is not Mac-specific. This was dropped in 3.11
• The -I and -E options respect it.

Along the way:

• Fix doc that PYTHONEXECUTABLE affects sys.executable, not sys.argv[0]

Fixes #133525

• Issue: PYTHONEXECUTABLE envvar incorrectly doc'd as mac-only and ignored by -E/-I #133525

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📚 Documentation preview 📚: https://cpython-previews--133526.org.readthedocs.build/

[python-cla-bot]

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[StanFromIreland]

Please fix the styling, it should be:

:envvar:`PYTHONEXECUTABLE`

and incline code should use double backticks i.e.

``-E``

[rickeylev]

Thanks @StanFromIreland !

Done: Added the sphinx roles to the backtick'd symbols. Instead of double backticks for -E, -I, I added :option: to them.

PTAL

[rickeylev]

Oops, thanks for catching that. Suggestion applied.