Created by: cvrebert
Fixes #14038 (closed) by using <a tabindex="0"> instead of <button>.
Demo: http://jsfiddle.net/v5L7m/3/
Supporting research: https://gist.github.com/cvrebert/68659d0333a578d75372
Also moves the "Multiple-line links" callout together with the rest of the general Popover callouts.
Also switches from {trigger: 'focus'} to data-trigger="focus".