Do not assume module location, use Drupal functions to get the module path

Version 8.x-1.6 includes example colorbox styles in the following code in ColorboxSettingsForm.php:

  $form['colorbox_custom_settings']['colorbox_style'] = [
      '#type' => 'select',
      '#title' => $this->t('Style'),
      '#options' => $colorbox_styles,
      '#default_value' => $config->get('custom.style'),
      '#description' => $this->t('Select the style to use for the Colorbox. The example styles are the ones that come with the Colorbox plugin. Select "None" if you have added Colorbox styles to your theme.  <br> <strong>Examples</strong>: <ul><li><a href="/modules/contrib/colorbox/images/admin/example_default.png" target="blank">Default</a></li><li><a href="/modules/contrib/colorbox/images/admin/example_plain.png" target="blank">Plain</a></li><li><a href="/modules/contrib/colorbox/images/admin/example_stockholm_syndrome.png" target="blank">Stockholm Syndrome</a></li><li><a href="/modules/contrib/colorbox/images/admin/colorbox_example_1.png" target="blank">Example 1</a></li><li><a href="/modules/contrib/colorbox/images/admin/colorbox_example_2.png" target="blank">Example 2</a></li><li><a href="/modules/contrib/colorbox/images/admin/colorbox_example_3.png" target="blank">Example 3</a></li><li><a href="/modules/contrib/colorbox/images/admin/colorbox_example_4.png" target="blank">Example 4</a></li><li><a href="/modules/contrib/colorbox/images/admin/example_none.png" target="blank">None</a></li></ul>'),
    ];

Drupal modules should never assume the location of module directories. For example, many sites use a multisite setup or place modules in a subdirectory such as /modules/. In those types of environments, the #description will always return a "404 Not Found".

Drupal 7 issue: https://www.drupal.org/project/colorbox/issues/3127345