Remove specific use case example in documentation

It probably suffices to just say "under certain circumstances" here.
Mention parallelization plug-ins in async section

Provides an explanation for some uses of the async option, including
alternatives like the grunt-concurrent plug-in.
Improve async/asyncOutput messages

With regards to advising the user how to debug the asynchronous run
action, clarifies the flags available, and with asyncOutput enabled,
states that output will follow.