[llvm] [LangRef] make consequences of NaN rules for pow(i) more explicit (PR #170177)

[James Y Knight via llvm-commits]

jyknight wrote:

> My biggest concern is that the eventual move away from constrained intrinsics will require modification of this wording, but it's probably best to deal with that when we make that move rather than trying to do the wording for that scenario right now.

Yes, we should not word it for the eventual future, but for the now.  And making the semantics clearer now will help ensure everyone's on the same page as to the semantics we need to preserve in such future-state.

https://github.com/llvm/llvm-project/pull/170177