[TD-5606] <docs>: change default value of "numOfMnodes" to be 1.

documentation20/cn/11.administrator/docs.md

@@ -143,7 +143,7 @@ taosd -C
 
 TDengine集群中加入一个新的dnode时，涉及集群相关的一些参数必须与已有集群的配置相同，否则不能成功加入到集群中。会进行校验的参数如下：
 
-- numOfMnodes：系统中管理节点个数。默认值：3。
+- numOfMnodes：系统中管理节点个数。默认值：3。（2.0 版本从 2.0.20.11 开始、2.1 及以上版本从 2.1.6.0 开始，numOfMnodes 默认值改为 1。）
 - mnodeEqualVnodeNum: 一个mnode等同于vnode消耗的个数。默认值：4。
 - offlineThreshold: dnode离线阈值，超过该时间将导致该dnode从集群中删除。单位为秒，默认值：86400*10（即10天）。
 - statusInterval: dnode向mnode报告状态时长。单位为秒，默认值：1。

documentation20/en/11.administrator/docs.md

@@ -132,7 +132,7 @@ The SQL creates a database demo, each data file stores 10 days of data, the memo
 
 When adding a new dnode to the TDengine cluster, some parameters related to the cluster must be the same as the configuration of the existing cluster, otherwise it cannot be successfully added to the cluster. The parameters that will be verified are as follows:
 
-- numOfMnodes: the number of management nodes in the system. Default: 3.
+- numOfMnodes: the number of management nodes in the system. Default: 3. (Since version 2.0.20.11 and version 2.1.6.0, the default value of "numOfMnodes" has been changed to 1.)
 - balance: whether to enable load balancing. 0: No, 1: Yes. Default: 1.
 - mnodeEqualVnodeNum: an mnode is equal to the number of vnodes consumed. Default: 4.
 - offlineThreshold: the threshold for a dnode to be offline, exceed which the dnode will be removed from the cluster. The unit is seconds, and the default value is 86400*10 (that is, 10 days).