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

[J. Ryan Stinnett via llvm-commits]

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@@ -187,6 +192,10 @@ Optimizations
    This is a migration guide describing how to move from debug info using
    intrinsics such as dbg.value to using the non-instruction DbgRecord object.
 
+:doc: `KeyInstructionsDebugInfo`
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jryans wrote:

Same question about the space here as well.

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