Document that dropdowns are intentionally click-based rather than hover-based

And link to a relevant blog post w/ more info.
Refs #16966
[skip sauce]

docs/components/dropdowns.md

@@ -4,7 +4,7 @@ title: Dropdowns
 group: components
 ---
 
-Dropdowns are toggleable, contextual overlays for displaying lists of links and more. They're made interactive with the included Bootstrap dropdown JavaScript plugin.
+Dropdowns are toggleable, contextual overlays for displaying lists of links and more. They're made interactive with the included Bootstrap dropdown JavaScript plugin. They're toggled by clicking, not by hovering; this is [an intentional design decision.](http://markdotto.com/2012/02/27/bootstrap-explained-dropdowns/)
 
 ## Contents