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

[YunQiang Su via llvm-commits]

wzssyqa wrote:

> You want to restrict it so that it can't return -0 if both arguments are +0. I don't think that's going to happen, but I do think the current definition of nsz allows it.

In fact it may happen. I had a try to expand `FMINIMUM` with `fadd OP, 0.0`.
And finally I found that `FMINIMUM -0.0, -0.0` may return `+0.0`.
So I prefer to state  explicitly that it is not allowed.

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