Backport #152596 (Pointer authentication documentation) to llvm21

[ojhunt]

/cherry-pick 62d2a8e

[llvmbot]

Failed to create pull request for issue154236 https://github.com/llvm/llvm-project/actions/runs/17056056901

[ojhunt]

Testing GH discover of the high 62d2a8e

[ojhunt]

Backport #62d2a8e ?

[ojhunt]

Or the issue number: #152596

[EugeneZelenko]

/cherry-pick 62d2a8e

[llvmbot]

Failed to create pull request for issue154236 https://github.com/llvm/llvm-project/actions/runs/17056056901