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

[Matt Arsenault via llvm-commits]

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@@ -16831,6 +16836,11 @@ trapping or setting ``errno``.
 When specified with the fast-math-flag 'afn', the result may be approximated
 using a less accurate calculation.
 
+Note that due to how :ref:`LLVM treats NaN values <floatnan>`, the special cases
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arsenm wrote:

Oh, I see IEEE has the signaling nan case explicitly called out in the pow cases unlike the libm docs 

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