v4 - Accordion example uses v3 panel markup

[zacechola]

http://v4-alpha.getbootstrap.com/components/collapse/#accordion-example

[tshinnic]

Just noticed this now in recent sources, 2 months later, so would be nice to see this PR merged, especially if alpha2 is imminent.

However, and without trying @kkirsche 's fix, why does the Accordion example work without any of these old or new classes? They are only for the styling of appearance?

[tshinnic]

Hmm, apparently not just styling, some behavior too. As it was, with no classes operative, the accordion was a single pane open at a time. If you clicked on another group item, it expanded while closing the previously open item. With kkirsche's fix, now you can open/close each group independently. This is radio button vs. checkbox behavior.

There are actually two behaviors possible - does the documentation say which behavior is available, or how to get either behavior?

[cvrebert]

Duplicate of #18375.