writeback, blkio: add documentation for cgroup writeback support

Update Documentation/cgroups/blkio-controller.txt to reflect the
recently added cgroup writeback support.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: cgroups@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>