[clang] [llvm] [KeyInstr] Add docs (PR #137991)

[Orlando Cazalet-Hyams via llvm-commits]

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+# Key Instructions debug info in LLVM
+
+Key Instructions reduces the jumpiness of optimized code debug stepping. This document explains the feature and how it is implemented in LLVM. For Clang support please see the [Clang docs](../../clang/docs/KeyInstructionsClang.md)
+
+## Status
+
+In development, but mostly complete. The feature is currently disabled for coroutines.
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OCHyams wrote:

I guess it essentially is "random"; it's just that at the time of writing, and the branch date, I haven't got to them yet. There's no fundamental reason why they're not supported and we intend to get to them soon. Mixing functions with different key-instructions-modes is fully supported - it just means currently coros use the non-key-instructions handling.

I've added a bit to the sentence, how does this sound?

https://github.com/llvm/llvm-project/pull/137991