Remove unnecessary std::move in return statements...

... and comment where it is necessary.

src/ondemand/ODComputeSummaryTask.cpp

@@ -39,6 +39,7 @@ movable_ptr<ODTask> ODComputeSummaryTask::Clone() const
 {
    auto clone = make_movable<ODComputeSummaryTask>();
    clone->mDemandSample = GetDemandSample();
+   // This std::move is needed to "upcast" the pointer type
    return std::move(clone);
 }