Filter the Additional Information Placeholder Text field in WooCommerce

You can filter the Additional Information placeholder text field in WooCommerce checkout page with the woocommerce_checkout_fields filter.

Use it in functions.php like so…

add_filter( 'woocommerce_checkout_fields' , 'wpb_custom_additional_info' );
// Change placeholder text in Additional Notes
function wpb_custom_additional_info( $fields ) {
     $fields['order']['order_comments']['placeholder'] = 'Add a Shipping address here if different from Billing Address, or any additional notes for delivery';
     return $fields;
}

Ref