SPARK-4040. Update documentation to exemplify use of local (n) value, fo...

This is a minor docs update which helps to clarify the way local[n] is used for streaming apps.  
I actually though the default behavior of `local` was 2 threads, but in the code I don't think that's true. I thought @mateiz mentioned one time that it's better to run with minimal parallelism by default to expose issues that might only appear when there are multiple executors.

In any event, given that, and the thrust of this doc change, is it good to encourage people to use 1 worker? how about explicitly 2? Making it explicit is a small good thing anyway.