v4 - `btn-group-xs` not working

Created by: Qqwy

http://v4-alpha.getbootstrap.com/components/button-group/#sizing

In above section, the <div class="btn-group btn-group-xs" role="group" aria-label="...">...</div> shows up with the same size as the normal button group.

Either btn-group-xs is something that ought to be added and is not yet in the new Bootstrap alpha(I hope so, it is a great feature), or this section in the documentation needs updating.