The help text in general (both field descriptions and actual help page) does not conform to UI guidelines and needs clean-up. Specifically, it's all way too long, and it focuses on exposing the underlying technical details vs. being more use-case focused "Use this to foo your bar so your users can baz." See Field UI settings descriptions for an example.
User interface changes
There are no API changes: those have already been taken care of in other issues. This would just improve the ability to make use of those API changes through a UI.
Beta phase evaluation
|Issue category||Task because this a follow-up to a recent critical change.|
|Issue priority||Major because the sizes attribute will be very difficult to configurate without this UI change.|
|Prioritized changes||The main goal of this issue is improve usability for the new sizes attribute. Making this easier to use will give Drupal 8 full ability to make use of the responsive images specification, which ultimately means better front-end performance for Drupal 8 sites. This is also a follow-up change to a recent critical,.|
|Disruption||This will not be disruptive for core, contrib modules or themes. This only adds easier configuration and does not change the underlying APIs.|