Allow disabling of individual fields within the field override UI

Retitling for clarity. And changing version because new features only go into 2.0.x.

We are talking about the field override UI here. Disabling fields via form_alter already works via #after_build:

  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['address'] = [
      '#type' => 'address',
      '#default_value' => [
        'country_code' => 'US',
        'administrative_area' => 'CA',
        'locality' => 'Mountain View',
        'postal_code' => '94043',
        'address_line1' => '1098 Alta Ave',
        'organization' => 'Google Inc.',
        'given_name' => 'John',
        'family_name' => 'Smith',
      ],
      '#after_build' => [[get_class($this), 'disableFields']],
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];

    return $form;
  }

  /**
   * #after_build callback: Disables certain fields.
   */
  public static function disableFields(array $element, FormStateInterface $form_state) {
    $element['address_line2']['#attributes']['disabled'] = 'disabled';

    return $element;
  }

We should add this example to the documentation.