gh-56229: Document flush expectation for stdout and stderr replacements

[ahmojo]

Summary

Document that replacement sys.stdout and sys.stderr objects should provide flush() because these streams are flushed during interpreter shutdown.

What changed

• Added one focused sentence to Doc/library/sys.rst.
• No behavior changes.
• No new dependencies.

Tests

• git diff --check
• Doc\make.bat check
• Full Sphinx HTML build with --fail-on-warning

Closes #56229

• Issue: [doc] Attribute error with flush on stdout,stderr #56229

[python-cla-bot]

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[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33443320 | 📁 Comparing 5df1d7d against main (8e88bb5)

  🔍 Preview build  

1 file changed

± library/sys.html