Update Documentation/cgroups/blkio-controller.txt to reflect the
recently added cgroup writeback support.
Signed-off-by: NTejun 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: NJens Axboe <axboe@fb.com>