*.libraries.yml, but didn't document the format. There are also some other *.yml files with no documentation on api.drupal.org
Generally, the idea for API documentation in Drupal Core is:
- Make sure there is a topic (@defgroup in a core api.php file) that gives you the bare minimum documentation for that API -- not necessarily all the details, but at least the basic syntax/usage/etc.
- Make sure the topic is linked from the landing page on api.drupal.org for Drupal 8, or that there is a logical path to go through to find it (e.g., if you were documenting the API for making menu links, it should either be in a topic with "menu" in the name that is listed on the api.d.o landing page, or if you go to the "menu something" topic that is listed there, that page should link to this other topic that has this information in it).
- From the "bare minimum" @defgroup topic, link to more detailed documentation on d.o. The page on d.o should give you more details, in more of a tutorial style.
The *.yml files in Core are listed in comment # 26. Most of them are documented OK, but there are 4 child issues that need to be resolved. Each one is rounding out or adding documentation for one of these *.yml files.
User interface changes