Issue #2137223 by YesCT: Config translation code documentation and small code clean up.

...dules/config_translation/css/config_translation.admin.css

...ig_translation/Access/ConfigTranslationOverviewAccess.php

...ig_translation/Controller/ConfigTranslationController.php

...ntroller/ConfigTranslationFieldInstanceListController.php

...ranslation/Controller/ConfigTranslationListController.php

...ig_translation/Controller/ConfigTranslationMapperList.php

...l/config_translation/Form/ConfigTranslationDeleteForm.php

...pal/config_translation/Form/ConfigTranslationEditForm.php

...pal/config_translation/Form/ConfigTranslationFormBase.php

...ion/lib/Drupal/config_translation/FormElement/Element.php

...on/Plugin/Derivative/ConfigTranslationContextualLinks.php

...slation/Plugin/Derivative/ConfigTranslationLocalTasks.php