fixes #25343 by mentioning how collapse animates in docs

docs/4.0/components/collapse.md

@@ -6,6 +6,10 @@ group: components
 toc: true
 ---
 
+## How it works
+
+The collapse JavaScript plugin is used to show and hide content. Buttons or anchors are used as triggers that are mapped to specific elements you toggle. Collapsing an element will animate the `height` from it's current value to `0`. Given how CSS handles animations, you cannot use `padding` on a `.collapse` element. Instead, use the class as an independent wrapping element.
+
 ## Example
 
 Click the buttons below to show and hide another element via class changes: