docs(popover): add tips to use get-container in popover (#10580)

src/popover/README.md

@@ -155,6 +155,8 @@ export default {
 };
 ```
 
+> Tips: When use get-container, the mount location cannot be the reference slot node of popover
+
 ## API
 
 ### Props

src/popover/README.zh-CN.md

@@ -195,6 +195,8 @@ export default {
 };
 ```
 
+> 注意：使用 get-container 时，挂载的父节点不可以是 popover 的 reference slot 节点
+
 ## API
 
 ### Props