Take a #config_target form element's #title and #description from its config schema

Problem/Motivation

A setting's label and description are often written twice: as label in its config schema, and as #title and #description on its settings form element.

Example: for the page cache's maximum age system.schema.yml reads:

max_age:
  type: integer
  label: 'Max age'

And PerformanceForm reads:

$form['caching']['page_cache_maximum_age'] = [
  '#type' => 'select',
  '#title' => $this->t('Browser and proxy cache maximum age'),
  '#config_target' => 'system.performance:cache.page.max_age',
  '#options' => $period,
  '#description' => $this->t('This is used as the value for max-age in Cache-Control headers.'),
];

There a number of challenges with the code above. They often drift (e.g. 'Max age' vs 'Browser and proxy cache maximum age') but perhaps more importantly, the form fields can't be accessed by other users inside or outside of Drupal. This issue was split out of #3635315: [Prototype] Write a setting's rules once, in config schema, and use them for validation, forms and the published JSON Schema where I go into a lot more detail about these challenges.

With the MR, the schema looks as follows:

max_age:
  type: integer
  label: 'Browser and proxy cache maximum age'
  description: 'This is used as the value for max-age in Cache-Control headers.'

And the form element can leave out the title and description:

$form['caching']['page_cache_maximum_age'] = [
  '#type' => 'select',
  '#config_target' => 'system.performance:cache.page.max_age',
  '#options' => $period,
];

The text now lives in one place, so the form and the schema can no longer drift apart. And because the label and description moved out of the form's PHP code and into its schema, other code (e.g.ECA, FlowDrop, Maestro, Tool API, etc) can describe a setting the same way a Drupal form does.

#3477363: Add a way to automatically generate basic config forms goes further: it generates whole form elements from config schema. This issue helps us get a step closer to making #3477363 reality.

In #3477363, joachim and james.williams agreed that a description belongs in config schema (https://www.drupal.org/project/drupal/issues/3477363#comment-16098165).