null_blk: add an usage for shared tags in documentation

Add usage explanation for a shared_tags, introduced by commit:

82f402fe ("null_blk: add support for shared tags")

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>

Reworded slightly.

Signed-off-by: Jens Axboe <axboe@kernel.dk>