Adding description to archive of Custom Post Type

Im battling to add a description / introduction field to the top of the archive of my custom post type. I manage easy with with ACF’s option page, but I would like to do without ACF.

I have managed, halfway, but i think my code is messy…
The description/introduction does displays on the frontend, but in the admin, the textarea’s layout is not funny: I can’t get it to display full-width and some odd spaces are added once I hit save.
Please help me out 🙂

See 2x screenshots.

Is there a better way to simply add a custom field where users can add a description which will the display at the top of custom post type page;
Very much the same as one can add a description for a taxonomy.

This is my code:

//http://wpsettingsapi.jeroensormani.com/
add_action( 'admin_menu', 'tsumtour_add_admin_menu' );
add_action( 'admin_init', 'tsumtour_settings_init' );
function tsumtour_add_admin_menu(  ) { 
    add_submenu_page( 'edit.php?post_type=tour', 'Tour', 'Intro', 'manage_options', 'tour', 'tsumtour_options_page' );
}
function tsumtour_settings_init(  ) { 
    register_setting( 'pluginPage', 'tsumtour_settings' );
    add_settings_section(
        'tsumtour_pluginPage_section', 
        __( 'Introduction', 'tsum_tour' ), 
        'tsumtour_settings_section_callback', 
        'pluginPage'
    );
    add_settings_field( 
        'tsumtour_textarea_field_0', 
        __( '', 'tsum_tour' ), 
        'tsumtour_textarea_field_0_render', 
        'pluginPage', 
        'tsumtour_pluginPage_section' 
    );
}
function tsumtour_textarea_field_0_render(  ) { 
    $options = get_option( 'tsumtour_settings' );
    ?>
    <textarea cols="80" rows="15" name="tsumtour_settings[tsumtour_textarea_field_0]"> 
        <?php echo $options['tsumtour_textarea_field_0']; ?>
    </textarea>
    <?php
}