docs: add a guide to annotate component type in decorator

[ktsn]

This PR adds a new docs section that describes how to annotate this type on functions in the decorator.

@Component<Post>({ // <---
  watch: {
    postId(id: string) {
      this.fetchPost(id)
    }
  }
})
class Post extends Vue {
  postId: string

  fetchPost(postId: string): Promise<void> {
    // ...
  }
}