blk-wbt: Eliminate ambiguity in the comments of struct rq_wb

In the current implementation, the last_issue and last_comp members of
struct rq_wb are used only by read requests and not by non-throttled write
requests. Therefore, eliminate the ambiguity here.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20250727173959.160835-3-yizhou.tang@shopee.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Tang Yizhou

block/blk-wbt.c

@@ -85,8 +85,8 @@ struct rq_wb {
 	u64 sync_issue;
 	void *sync_cookie;
 
-	unsigned long last_issue;		/* last non-throttled issue */
-	unsigned long last_comp;		/* last non-throttled comp */
+	unsigned long last_issue;	/* issue time of last read rq */
+	unsigned long last_comp;	/* completion time of last read rq */
 	unsigned long min_lat_nsec;
 	struct rq_qos rqos;
 	struct rq_wait rq_wait[WBT_NUM_RWQ];