How can I add an area/option for a custom page description?

A simple, easy, and quick way to add your own custom meta box to your Pages is by first installing the Meta Boxes plugin (required). Next, add the following custom code to your functions.php in your child theme:

add_filter( 'rwmb_meta_boxes', 'wpse_236870_meta_boxes' );

function wpse_236870_meta_boxes( $meta_boxes ) { // Add custom meta box in 'Pages'
    $meta_boxes[] = array(
        'title' => __( 'Some Meta Box Title', 'textdomain' ),
        'post_types' => 'page',
        'fields' => array(
            array( // Add custom textbox
                'id'   => 'your_id_here',
                'name' => __( 'Some Title', 'textdomain' ),
                'type' => 'textarea',
            )
        ),
    );
    return $meta_boxes;
}

As a result, you have your custom meta box.

The authors of the Meta Boxes plugin have a very extensive documentation which will help you with anything else.