blk-mq: docs: add kernel-doc description for a new struct member

As reported by kernel-doc:
	./include/linux/blk-mq.h:267: warning: Function parameter or member 'active_queues_shared_sbitmap' not described in 'blk_mq_tag_set'

There is now a new member for struct blk_mq_tag_set. Add a
description for it, based on the commit that introduced it.

Fixes: f1b49fdc ("blk-mq: Record active_queues_shared_sbitmap per tag_set for when using shared sbitmap")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/8e513153b83eefc05e358f51f2632b592c3f6772.1603791716.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>