Updated Async scenarios

I've updated
http://www.pacificspirit.com/Authoring/async/async-scenarios.html

 

I did most of David Hull's comments, added in 2 of the 3 appendix
scenarios, added the wsa marker, added a couple fault variants, added
some notes, added a purpose section.

 

Still a bit more work to be done, but I think it's good enough for
people to make decisions...

 

Cheers,

Dave