blk-zoned: Improve the queue reference count strategy documentation

blk-zoned: Improve the queue reference count strategy documentation

For the blk_queue_exit() calls, document where the corresponding code can
be found that increases q->q_usage_counter.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20241217210310.645966-4-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>