Doc: Mark SIGSTOP as unavailable on Windows

[nmurrell07]

Summary

This PR adds an availability note to the SIGSTKFLT signal documentation, clarifying that it's Unix-specific.

Note: There appears to be a mismatch between this issue's title ("Better query string parser") and the actual diff (Python signal documentation). The original issue was marked invalid since Node.js implemented their own query string parser. This change is unrelated to query string parsing — please clarify if this should be associated with a different issue or if there's been a mix-up in the PR context.

What Changed

Added .. availability:: Unix. directive to the SIGSTKFLT signal documentation entry:

.. data:: SIGSTKFLT

   Stack fault on coprocessor. The Linux kernel does not raise this signal: it

+   .. availability:: Unix.

Why This Matters

The signal module documentation should clearly indicate platform-specific signals. SIGSTKFLT is only available on Unix systems, and this note helps developers understand where they can expect to use it (or not).

Verification

• Reviewed Python's signal module source code to confirm SIGSTKFLT availability
• Checked existing documentation patterns for other Unix-only signals
• Verified the reStructuredText syntax matches project conventions

Limitations

⚠️ Important: This change appears disconnected from the original issue about query string parsing. The maintainer should verify:

1. Whether this is the correct PR for this issue
2. If there's a different issue number this should reference
3. Whether additional context was lost in transit

If this is indeed unrelated to the "query string parser" issue, please let me know so I can update accordingly.

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📚 Documentation preview 📚: https://cpython-previews--146322.org.readthedocs.build/

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[picnixz]

Dupe of #146319