Document and test WarpScan::Scan's treatment of `init` argument

```cpp
...
int input = 1;
int init = 10;
int inclusive_output, exclusive_output;

using WarpScan = cub::WarpScan;
__shared__ typename WarpScan::TempStorage storage[warps_no];
WarpScan(storage[warp_id])
.Scan(input, inclusive_output, exclusive_output, init, cub::Sum());
...
```
Should `inclusive_output` after calling `WarpScan::Scan(..)` function include `init`? Right now it does, but `WarpScan::Scan`'s documentation is unclear about it.