Expanded comments on reopenClass

This provides further detail on the reopenClass method. When I just starting to look at Ember, I found several online examples using reopenClass (including http://eviltrout.com/2013/03/23/ember-without-data.html), and the method confused me (and there was usually someone asking about the same thing in the comments). 

I know from a pure Javascript prospective, the description "Augments a constructor's own properties and functions" is technically correct. But describing them instead as static properties was much more informative (and familiar).