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

[J. Ryan Stinnett via llvm-commits]

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@@ -102,6 +103,10 @@ Clang
 :doc:`CFIVerify`
   A description of the verification tool for Control Flow Integrity.
 
+:doc: `KeyInstructionsClang`
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jryans wrote:

Does this render correctly as a link to your page...? None of the other lines have a space between `:doc:` and the page name, but maybe it works...?

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