docs(ngBindHtml): Added important info to comment

Added comment. It doesn't say you need to include angular-sanitize.js otherwise ngSanitize will cause an error!
Yeah sure, as long as there is some info for people. I probably needed like an hour to figure out how to print out html in ng-repeat, which shouldn't be a big deal imo. I'm a beginner, so couldn't understand why such a small thing was so hard to achieve.