Consider adding descriptions to config schema

#1: AFAICT @tstoeckler intended for it to be used in the "add/edit translation" config translation forms: \Drupal\config_translation\Form\ConfigTranslationAddForm and friends. That would provide context to the person working on translations.

In a next step, we could remove the hardcoded descriptions from config forms. See the attached PoC patch as an example

But by now, I think it'd also be valuable for API responses and generating config forms in general, which would enable decoupled admin UIs. See 🌱 [META] Untie config validation from form validation — enables validatable Recipes, decoupled admin UIs … Active .