[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
> 
> I think the problem is alive2's interpretation of nsz maxnum(+0, +0) assumes -0 is a valid output

It is a valid output, isn't it?

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