config in nacos：seata.service.vgroup-mapping modification message push

## 0. Question
Is the modification message push description in the [doc](http://seata.io/zh-cn/docs/user/txgroup/transaction-group-and-ha.html) has already achieved? I should do it by myself ?

## 1. Prepare
I have a seata-server cluster named 'default' and the client config following before start up seata-client.
```yaml
seata:
service:
vgroup-mapping:
my_test_tx_group: default1
```
## 2. Running
I start up my client. Of course it has such log:
```log
2021-07-09 15:25:51.419 ERROR 89321 --- [eoutChecker_1_1] i.s.c.r.netty.NettyClientChannelManager : no available service found in cluster 'default1', please make sure registry config correct and keep your seata server running
```

## 3. Modify Config
According to the [doc](http://seata.io/zh-cn/docs/user/txgroup/transaction-group-and-ha.html), modify the **my_test_tx_group** to **default**. But I still get the error log
```log
2021-07-09 15:42:31.429 ERROR 89321 --- [eoutChecker_2_1] i.s.c.r.netty.NettyClientChannelManager : no available service found in cluster 'default1', please make sure registry config correct and keep your seata server running
```

Start with the linked transaction-group and HA documentation and reproduce the reported configuration change using the shown `seata.service.vgroup-mapping` YAML and client error logs. Trace how the client receives configuration updates and confirm whether changing `my_test_tx_group` from `default1` to `default` is reflected; done means the behavior is explained and documented or a verified fix is identified.

Written by the indexing model from the issue text.