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

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams commented:

Thanks for for the comments, I think I've addressed everything. I've merged both files into the LLVM docs as I couldn't get a link to the Clang docs working properly. How does this look?

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