[cpp-threads] Web site updated

[Hans Boehm]

I updated the web site at

http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/

In particular, I finally added the promised rationale documents under the
seventh bullet:

    * Why do we leave semantics for races on ordinary variables completely
undefined?
    * Why do our atomic operations have integrated memory ordering
specifications?
    * Why do we not guarantee that dependencies enforce memory ordering?
    * Why do our ordering constraints not distinguish between loads and
stores, when many architectures provide fences that do?

Hans