Improve Migration Doc for .help-block and .form-group

Created by: tordans

I could not find those things in the migration doc but I think they should be added:

• ".help-block" the change is documented at http://v4-alpha.getbootstrap.com/components/forms/#help-text but should be documented here as well.
• Section Forms: I don't understand this sentence: ".form-group no longer mixins the .row class, so it’s now required for grid layouts.". What exactly is "mixins" and what exactly should I change in my form html?