[Feature Request] Parameter of change event emitted by cascader is not consistent with the description

### Existing Component
Yes

### Component Name
Cascader

### Description
Accroding to the documentation, we could get the value of the node we select in a cascader when we set `emitPath` as `false` in `props`.
If I alse set `multiple` as `true` like this,
```js

```
We expected to get the latest selected value, but we got a array of all selected values instead.

Although we all know that is consistent with the design priciple of `@change` event, we have no any other ways to get the latest selected value in such a situation, setting `multiple` as `true` as well as `emitPath` as `false`.

If there were any approach we missed, please tell us. If no, we hope that could be a new feature in next update.