re-apply Timer linting fixes that happened in the tiled ui PR

src/UserActivity.js

@@ -44,6 +44,7 @@ class UserActivity {
      * Can be called multiple times with the same already running timer, which is a NO-OP.
      * Can be called before the user becomes active, in which case it is only started
      * later on when the user does become active.
+     * @param {Timer} timer the timer to use
      */
     timeWhileActive(timer) {
         // important this happens first

src/utils/Timer.js

@@ -70,6 +70,7 @@ export default class Timer {
 
     /**
      * if not started before, starts the timer.
+     * @returns {Timer} the same timer
      */
     start() {
         if (!this.isRunning()) {
@@ -81,6 +82,7 @@ export default class Timer {
 
     /**
      * (re)start the timer. If it's running, reset the timeout. If not, start it.
+     * @returns {Timer} the same timer
      */
     restart() {
         if (this.isRunning()) {
@@ -98,6 +100,7 @@ export default class Timer {
     /**
      * if the timer is running, abort it,
      * and reject the promise for this timer.
+     * @returns {Timer} the same timer
      */
     abort() {
         if (this.isRunning()) {