You would need to include the content in your global HTML template. Content in your HTML template is present on every page of your side however it can be hidden from all pages using CSS and then only displayed on the checkout page by targeting using the specific pages unique body id.
For example if you have added a div element with the id cart-text you can apply the following rules to hide it everywhere but show it on the checkout page.
The benefit of adding the content via the HTML template is that it's rendered along with the rest of the page whereas injected code is added client side after the page loads. Downside of course is that technically the content will be present on every page and just hidden from view on some pages.