This is the right place for discussing the documentation.
Yes, it is possible to add Latest Activity, Comments, or anything in different tabs. In the case of Activity, you might want to look at the sidebar_widget_activity method in the Activity class (plugins/activity/activity.php). You'd first need to check if that file exists, and then create an object of type Activity with $act = new Activity($hotaru); Then you can use $act to call other methods like $act->getSerializedSettings('activity'); $act->getLatestActivity(10); and $act->getSidebarActivityItems($activity, $activity_settings);
Note that from Hotaru 0.9, this line in the documentation will be changed from:
$sbp = new SidebarPosts('sidebar_posts', $hotaru); // provides access to SidebarPosts functions
$sbp = new SidebarPosts($hotaru); // provides access to SidebarPosts functions