make expanding explanation accessible

See also: #4600 LMS-11161 https://openedx.atlassian.net/browse/LMS-11161 
@cptvitamin, when the markup for this was first built (by @frrrances I believe), the philosophy was to progressively enhance the UI so that the content (in this case the heading and the copy corresponding to it) was marked up semantically and then JavaScript would add the collapse/expand behavior on top of that. I'm not sure if the implementation has changed or not since then.