blk-mq: Explain when 'active_queues' is decremented

It is nontrivial to derive from the blk-mq source code when
blk_mq_tags.active_queues is decremented. Hence add a comment that
explains this.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>