Rollup merge of #159819 - Vastargazing:fix/poison-error-auto-cfg-note, r=GuillaumeGomez

std::sync::poison: disable auto_cfg on PoisonError::new

`PoisonError::new` is defined twice, once for `#[cfg(panic = "unwind")]` and once for its negation. While exactly one definition survives expansion in any given build, the method itself is actually present in every configuration.

Currently, rustdoc's `auto_cfg` only looks at whichever `cfg` survived expansion and tags the method accordingly, generating a misleading "Available on panic=unwind only" portability badge in the docs.

This is the same `auto_cfg` limitation previously addressed in rust-lang/rust#153964, rust-lang/rust#154311, and rust-lang/rust#156426: when an item is split into mutually-exclusive `cfg` variants that collectively cover all builds, rustdoc incorrectly marks it as conditionally available. Issue rust-lang/rust#149786 pointed out `PoisonError::new` as another case of this pattern, but it hadn't been touched yet.

This PR applies the same fix as rust-lang/rust#156426 (adding `#[doc(auto_cfg = false)]` to both variants) and adds a regression test to cover this pattern going forward.

r? @GuillaumeGomez
tree: 66f8ecb6d4e0fec8cbf6628bb09d0cdd1c3e3136
  1. .github/
  2. compiler/
  3. library/
  4. LICENSES/
  5. src/
  6. tests/
  7. .clang-format
  8. .editorconfig
  9. .git-blame-ignore-revs
  10. .gitattributes
  11. .gitignore
  12. .gitmodules
  13. .ignore
  14. .mailmap
  15. bootstrap.example.toml
  16. Cargo.lock
  17. Cargo.toml
  18. CODE_OF_CONDUCT.md
  19. configure
  20. CONTRIBUTING.md
  21. COPYRIGHT
  22. INSTALL.md
  23. LICENSE-APACHE
  24. license-metadata.json
  25. LICENSE-MIT
  26. package.json
  27. README.md
  28. RELEASES.md
  29. REUSE.toml
  30. rust-bors.toml
  31. rustfmt.toml
  32. triagebot.toml
  33. typos.toml
  34. x
  35. x.ps1
  36. x.py
  37. yarn.lock
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