[skip ci] Add comment for scheduler's taskChan (#9686)

Signed-off-by: xige-16 <xi.ge@zilliz.com>

internal/querycoord/task_scheduler.go

@@ -45,6 +45,7 @@ type TaskQueue struct {
 	sync.Mutex
 }
 
+// Chan returns the taskChan of taskQueue
 func (queue *TaskQueue) Chan() <-chan int {
 	return queue.taskChan
 }