[skip ci]Add comments for rocksmq close (#9519)

Signed-off-by: fishpenguin <kun.yu@zilliz.com>

internal/util/rocksmq/server/rocksmq/rocksmq_impl.go

@@ -166,6 +166,10 @@ func NewRocksMQ(name string, idAllocator allocator.GIDAllocator) (*rocksmq, erro
 	return rmq, nil
 }
 
+// Close step:
+// 1. Stop retention
+// 2. Destroy all consumer groups and topics
+// 3. Close rocksdb instance
 func (rmq *rocksmq) Close() {
 	rmq.stopRetention()
 	rmq.storeMu.Lock()