[llvm] [IR] LangRef: document behavior of double-lifetime.start (PR #184296)

[Ralf Jung via llvm-commits]

RalfJung wrote:

I mean, Rust does emit them and there are plans to do it more in the future, so if they lead to miscompilations that'd be bad.^^

https://github.com/llvm/llvm-project/pull/184296