.accordion not explicitly mentioned in the docs

Created by: Herst

In v3 the accordion example didn't use any specific class (https://getbootstrap.com/docs/3.3/javascript/#collapse-example-accordion) now there is .accordion which isn't explicitly mentioned in the docs and just silently used in the code example with no explanation of what it's for (https://getbootstrap.com/docs/4.1/components/collapse/#accordion-example).