How to create notifications like Facebook about pending workflow approvals?
Have a look at one of the answers to "Notification + Subscription: Can this combination recipe be used to create notification system similar to Facebook&#39;s?", which explains how the Menu Badges module can be used for this. The result of using it looks as in this example:



The current status of this module for D8 is like so:


  The 8.x-1.x-dev release includes all the functionality of the Drupal 7 version. However, it is not yet completely working with the new caching that Drupal 8 does. Badges are getting cached and don't change when they should. Once that is fixed, a stable release will be created.


To implement it for supporting what you decribed in your update, you only have to create an appropriate view (using views) with as results the updated content that the editor should be notified about.