Well, that Help link is of no help whatsoever. It only references the Decorations area of the Paragraph panel; nowhere does it tell you how to add a line above or below any text. In fact, when I did it, the only thing it accomplished was to reduce the size of my selected text to 12 pt, and put a line under that. Then I had to go back and reset the font and size of the text to what it was before adding that line.
That's too much work, and definitely not intuitive. With InDesign and QuarkXPress, it's easy once you remember where the panel is in those programs.
Affinity needs to find a better, easier, intuitive way to do this.