Fix popover dismiss-on-next-click example

Fixes #14038 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"`.