message: Update documentation for GroupStore.registerListener
diff: @@ -167,11 +167,12 @@ class GroupStore extends EventEmitter {
  * immediately triggers an update to send the current state of the
  * store (which could be the initial state).
  *
  - * This also causes a fetch of all data of the specified group,
  - * which might cause 4 separate HTTP requests, but only if said
  - * requests aren't already ongoing.
  + * If a group ID is specified, this also causes a fetch of all data
  + * of the specified group, which might cause 4 separate HTTP
  + * requests, but only if said requests aren't already ongoing.
  *
  - * @param {string} groupId the ID of the group to fetch data for.
  + * @param {string?} groupId the ID of the group to fetch data for.
  + * Optional.
  * @param {function} fn the function to call when the store updates.
  * @return {Object} tok a registration "token" with a single
  * property `unregister`, a function that can
  