[llvm] [LLVM] Updated the LLVM CMake Documentation (PR #121083)

[Nikita Popov via llvm-commits]

https://github.com/nikic requested changes to this pull request.

I don't think the docs change is right. add_llvm_execute is for LLVM's own binaries, not for external projects.

https://github.com/llvm/llvm-project/pull/121083