We can't include anything besides basic (bold, italic, underline) formatting within a callout. However, callouts should be able to contain pretty much anything, such as tables, multiple paragraphs, and lists. I found an example of this here (https://tinymce.github.io/tinymce-demos/tinymce-basics/custom-toolbar-button-menu-and-context-toolbar.html) but to be fair, I don't know if the capability in that demo is a built-in thing, or an add-in.