Add about type inference of emits option into TS Documentation

[There is a section about props](https://v3.vuejs.org/guide/typescript-support.html#annotating-props) but there is no section about `emits` for now.

[active-rfcs/0030-emits-option](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0030-emits-option.md#type-inference)
