Let's say you are including styles that are coming from a third party and do not contain any DocBlock annotations. Still, you want to see these elements in the style guide too.
The solution for this is quite straigt-forward. Create empty dummy elements that match the vendored elements and put the annotations there. Since Nucleus is working with the SCSS source files, it is not important if a selector contains style declarations or not. In the end, after transpiling SCSS to CSS, the empty selector will not be in the CSS file.
/**
* I'm defining the style information for a
* vendored selector.
*
* @atom Bootstrap button
* @section Bootstrap
* @markup
* <button class="btn">a button</button>
*/
.btn { /* leave empty */ }