message: EventIndex: Document the return value of the isValidEvent method.
diff: @@ -247,6 +247,8 @@ export default class EventIndex extends EventEmitter {
  * or aren't of a type that we know how to index.
  *
  * @param {MatrixEvent} ev The event that should checked.
  + * @returns {bool} Returns true if the event can be indexed, false
  + * otherwise.
  */
  isValidEvent(ev) {
  const validEventType = ([
  