Refine comments

Author: Mel-Chen

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1658,8 +1658,9 @@ void VPlanTransforms::addActiveLaneMask(
 /// \p TypeInfo    VPlan-based type analysis.
 /// \p AllOneMask  The vector mask parameter of vector-predication intrinsics.
 /// \p EVL         The explicit vector length parameter of vector-predication
-///                intrinsics.
-/// \p PrevEVL     The explicit vector length of the previous iteration.
+/// intrinsics.
+/// \p PrevEVL     The explicit vector length of the previous iteration. Only
+/// required if \p CurRecipe is a VPInstruction::FirstOrderRecurrenceSplice.
 static VPRecipeBase *createEVLRecipe(VPValue *HeaderMask,
                                      VPRecipeBase &CurRecipe,
                                      VPTypeAnalysis &TypeInfo,