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

[jszuppe]

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

using WarpScan = cub::WarpScan<int, 32>;
__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.

[alliepiper]

We should update the docs and add a test, if needed.