[llvm] LangRef: Clarify behaviors of nsz in fast math flag (PR #137567)

[Andy Kaylor via llvm-commits]

andykaylor wrote:

Maybe I'm missing some context as to why this was needed, but I don't feel like the proposed changes clarifies anything. I find it to be less clear. If I am understanding it correctly, I think it would be sufficient to add just this:

`If the result is any zero, the returned value can be a zero of either sign.`

https://github.com/llvm/llvm-project/pull/137567