[LV] Document selectEpilogueVectorizationFactor (NFC).

Add missing documentation for selectEpilogueVectorizationFactor.

Suggested as independent improvement in D143938.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1199,6 +1199,9 @@ class LoopVectorizationCostModel {
   VectorizationFactor
   selectVectorizationFactor(const ElementCountSet &CandidateVFs);
 
+  /// \return The most profitable vectorization factor and the cost of that VF
+  /// for vectorizing the epilogue. Returns VectorizationFactor::Disabled if
+  /// epilogue vectorization is not supported for the loop.
   VectorizationFactor
   selectEpilogueVectorizationFactor(const ElementCount MaxVF,
                                     const LoopVectorizationPlanner &LVP);