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

[Jeremy Morse via llvm-commits]

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@@ -63,6 +63,7 @@ intermediate LLVM representation.
    ResponseGuide
    Remarks
    RemoveDIsDebugInfo
+   KeyInstructions
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jmorse wrote:

Is there a corresponding table-of-contents for clang too?

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