Re-add accidentally removed docs.

Author: NickGuy-Arm

llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h

@@ -139,6 +139,9 @@ 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.
+  /// Each element within Chains is 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,