Add Threadpool::TrySimpleParallelFor (#3759)
* Add TrySimpleParallerFor so that there's a path with OpenMP awareness for SimpleParallelFor. Makes it consistent with [Try]BatchParallelFor and [Try]ParallelFor.
Update TopK to check for the number of threads better, and to use TrySimpleParallelFor.
* Update doco to mention TrySimpleParallelFor
