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>

Documentation/block/null_blk.txt

@@ -77,3 +77,8 @@ use_lightnvm=[0/1]: Default: 0
 no_sched=[0/1]: Default: 0
   0: nullb* use default blk-mq io scheduler.
   1: nullb* doesn't use io scheduler.
+
+shared_tags=[0/1]: Default: 0
+  0: Tag set is not shared.
+  1: Tag set shared between devices for blk-mq. Only makes sense with
+     nr_devices > 1, otherwise there's no tag set to share.