Update to description of JEP 193: Enhanced Volatiles

[Paul Sandoz]

Hi,

I have, with the help of Brian, Doug and John, updated the description of JEP 193: Enhanced Volatiles to reflect our current thinking based on: 

- the prototype implementation in valhalla;
- performance measurements of that implementation; and
- investigations related to signature polymorphic methods and invokedynamic.

Thanks,
Paul.