[Feature Request] How to add or delete a child in el-table tree-table

### Existing Component
是

### Component Name
el-table

### Description
I don't find a way to add or delete a row of el-table tree-table in official document。
Is there any doc to help?

-------------

我在官方文档里没有找到添加或删除行的方法，有什么文档能提供帮助的么？