Add comment explaining TypeToFlatVector<Generic>

Summary:
This was a request in a comment made on 
#11170 I missed it before I landed the
change, so this is to address it.

Differential Revision: D63987591

velox/vector/VectorTypeUtils.h

@@ -85,6 +85,11 @@ struct TypeToFlatVector {
   using type = typename KindToFlatVector<SimpleTypeTrait<T>::typeKind>::type;
 };
 
+// Generic's, by design, do not have any compile time type information, so it is
+// impossible to determine what sort of Vector would hold values for this type.
+// To work around this, we just return BaseVector, since any Vector class can be
+// safely casted to BaseVector, and it is consistent with classes specialized
+// for the Generic type, like the VectorWriter.
 template <typename T, bool comparable, bool orderable>
 struct TypeToFlatVector<Generic<T, comparable, orderable>> {
   using type = BaseVector;