Document Entity API for Drupal 8

For someone evaluating whether the module is needed or not for a Drupal 7 site upgrade, it is unclear what features are in the Drupal 8 contrib module, that are not in core.

I started trying to track this down myself, gathering info from various parts:

• From: #2603890: [entity] Entity API and project homepage:
  

  The Drupal 8 core Entity API is significantly improved, so most of the features provided by the Drupal 7 version are already present in Drupal core.

  The Drupal 8 version of the Entity API module is used for improvements to Drupal 8's Entity API which will be moved to Drupal core one day (development ongoing).

• From README.txt on 8.x-1.x branch:
  

  @todo: Explain version compatibility pattern.
  @todo: Add overview of all items and maintainers.

  The "overview of all items" seems like a great place to start the doc improvements.

• From browsing the codebase:

  The doc blocks and inline code documentation is very thin, to the point that if you don't already know what's going on, scanning the code doesn't help very much.

• From scanning the issue queue:

  It is similarly very hard to figure out the status from reading issue titles, which are mostly related to tasks and bug fixes; only one of 13 open feature requests have been accepted (#2817751: Create an API for bundle plugins). If I understand correctly, that issue would let module developers / site builders write custom modules to create bundles and fields from code, instead of config. Could it be that this is really the *only* feature that's been implemented so far?