Updated SOV doc

[Brian Goetz]

I've updated the Language Model doc at

http://cr.openjdk.java.net/~briangoetz/valhalla/sov/02-object-model.html

to reflect the discussions surrounding abstract class supertypes and 
their use as reference projections, as well as the `ref-default` 
modifier.  Original version available here for those who want to try and 
diff them:

http://cr.openjdk.java.net/~briangoetz/valhalla/sov/02-object-model_01.html