集群扩展

地址：https://cn.dubbo.apache.org/zh-cn/docsv2.7/dev/impls/cluster/
问题：Cluster 接口 应该是 join 方法，不是merge 方法

Open the linked Cluster implementation documentation page and inspect the section describing the Cluster interface. Confirm that the method is named merge there, change the wording to join, and verify the rendered page shows the corrected method name.

Written by the indexing model from the issue text.