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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> It is OK to restrict the general rule to aligh the behavior with alive2 or to avoid some unwanted things. After all, there is no a specification for nsz, which would demand strictly defined behavior. The concern is that the documentation includes wording that cannot help a developer to properly handle nsz.

Alive2 needs for each instruction, a rule that indicates whether a given return value is valid or invalid.  The proposed wording is, as far as I can tell, minimal in that regard.

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