Modify the Additional CSS section (adding a disclaimer)

Yep. I was able to show it by default using:

<?php
add_action('customize_controls_print_styles', function()  {
?>
    <style id="custom-css">
        #sub-accordion-section-custom_css .description {
            display: block !important;
        }
    </style>
<?php
});

Thanks for your help!