Accordion docs and tests for multi_expand

This is a followup to https://github.com/zurb/foundation/pull/4577

However, I cannot get the second test ("multi-expand accordion should open the clicked section, leaving previous active panels open") to pass. I'm stumped, if it's something in the code that handles multi-expandable accordions, I can't figure out what, because the testing html markup works as expected when executed manually in a browser. Any thoughts?