Show comments from properties referenced via 'keyof T' #41220

// old way of writing events (since forever)
interface Evented1 {
  /** Handles the 'click' event. */
  on(type: "click", listener: (args: ClickEvent) => void): this;
  /** Handles the 'mousedown' event. */
  on(type: "mousedown", listener: (args: MouseDownEvent) => void): this;
  // ...
}

declare const obj1: Evented1;
obj1.on("click", _ => {}); // shows 'Handles the 'click' event in quickinfo when hovering over 'on'.
obj1.on("mousedown", _ => {}); // shows 'Handles the 'mousedown' event in quickinfo when hovering over 'on'.


// new way of events (used in lib.dom, may end up being used in Node at some point)...
interface Evented2EventMap {
  /** Handles the 'click' event. */
  click: ClickEvent;
  /** Handles the 'mousedown' event. */
  mousedown: MouseDownEvent;
  // ...
}

interface Evented2 {
  on<K extends keyof Evented2EventMap>(type: K, listener: (args: Evented2EventMap[K]) => void): this;
}

declare const obj2: Evented2;
obj2.on("click", _ => {}); // no comments when hovering over 'on'.
obj2.on("mousedown", _ => {}); // no comments when hovering over 'on'.

interface RequestMap {
  /** @deprecated Misspelled, use `AddItem` instead. */
  AddIetm: [AddItemRequest, AddItemResponse];

  /** Adds a new item to the collection */
  AddItem: [AddItemRequest, AddItemResponse];
}

interface EventMap {
  /** Raised whenever an item is added to the collection. **/
  CollectionChanged: CollectionChangedEventArgs;
}

interface Service {
  /** Sends a request to the service. */
  send<K extends keyof RequestMap>(type: K, args: RequestMap[K][0]): Promise<RequestMap[K][1]>;

  /** Listens for an event from the service. */
  on<K extends keyof EventMap>(type: K, listener: (args: EventMap[K]) => void): this;
}

declare const svc: Service;

service.send("AddItem", { }); 

service.on("CollectionChanged", args => {});