[LangRef] make consequences of NaN rules for pow(i) more explicit

Author: RalfJung

llvm/docs/LangRef.rst

@@ -16269,6 +16269,11 @@ Semantics:
 This function returns the first value raised to the second power with an
 unspecified sequence of rounding operations.
 
+Note that due to how :ref:`LLVM treats NaN values <floatnan>`, the special case
+of `powi(SNaN, 0.0)` can non-deterministically return *either* some NaN value
+(using the usual NaN propagation rules, so in particular the result could be
+either a signaling NaN or a quiet NaN), *or* the value `1.0`.
+
 .. _t_llvm_sin:
 
 '``llvm.sin.*``' Intrinsic
@@ -16831,6 +16836,12 @@ 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
+of `pow(1.0, SNaN)` and `pow(SNaN, 0.0)` can non-deterministically return
+*either* some NaN value (using the usual NaN propagation rules, so in particular
+the result could be either a signaling NaN or a quiet NaN), *or* the value
+`1.0`.
+
 .. _int_exp:
 
 '``llvm.exp.*``' Intrinsic