Descriptions can be added to a bundle page by navigating to the bundle's Design Editor From there, you will find the option to add a bundle description.

Navigate to your Bundles, click on the bundle you'd like to edit (or create a new one) and click on the 'Design editor' in the top right of your bundle.

Select a template that you think would work for you, and you'll be taken to the Design Editor, where you can add all of your content - including a description.

Depending on your template, the options you see may be different.