time_diff.description misses schema

Problem/Motivation

In TimestampFormatter

    $form['time_diff']['description'] = [
      '#type' => 'item',
    ];

item is '#input' => TRUE so this gets into config (opsie).

Steps to reproduce

Proposed resolution

Just add description to schema. Is it a good way to do it? Nope. It's not. But what else can be done? Much like the block issue, removing description from Views and such is too hard. And I do not see a submit method for FormatterInterface::settingsForm so it can't be removed even from the values array easily.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet