Docs: specify .card-inverse usage

Created by: maxxyme

Please specify in the Card documentation: (starting from) http://v4-alpha.getbootstrap.com/components/card/#image-overlays that .card-inverse only works for the following classes (as stated in bootstrap-flex.css): .card-header .card-footer .card-title .card-blockquote .card-link .card-text .card-subtitle .blockquote-footer