blk-mq: Fix a comment in include/linux/blk-mq.h

The 'hctx_list' member of struct blk_mq_hw_ctx is not a list head but
instead an entry in q->unused_hctx_list. Fix the comment above this
struct member.

Fixes: d386732b ("blk-mq: fill header with kernel-doc")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Cc: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk-mq.h

@@ -162,7 +162,10 @@ struct blk_mq_hw_ctx {
 	struct dentry		*sched_debugfs_dir;
 #endif
 
-	/** @hctx_list:	List of all hardware queues. */
+	/**
+	 * @hctx_list: if this hctx is not in use, this is an entry in
+	 * q->unused_hctx_list.
+	 */
 	struct list_head	hctx_list;
 
 	/**