Update doc for getScaledReductions

Author: NickGuy-Arm

llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h

@@ -139,9 +139,10 @@ class VPRecipeBuilder {
 
   /// Examines reduction operations to see if the target can use a cheaper
   /// operation with a wider per-iteration input VF and narrower PHI VF.
-  /// Returns null if no scaled reduction was found, otherwise a pair with a
-  /// struct containing reduction information and the scaling factor between the
-  /// number of elements in the input and output.
+  /// Recursively calls itself to identify chained scaled reductions.
+  /// Returns true if this invocation added an entry to Chains, otherwise false.
+  /// i.e. returns false in the case that a subcall adds an entry to Chains,
+  /// but the top-level call does not.
   bool getScaledReductions(
       Instruction *PHI, Instruction *RdxExitInstr, VFRange &Range,
       SmallVector<std::pair<PartialReductionChain, unsigned>> &Chains);