Document how to use MP Config to override context list with empty

[njr-11]

Our documentation for overrides via MicroProfile Config neglects to document how to use MicroProfile Config to clear our array properties (propagated context, cleared context, unchanged context). It turns out that MicroProfile Config is unclear on exactly how to do this. I've opened eclipse/microprofile-config#397 against MicroProfile Config for this. The main problem is disambiguating between an empty array and an array of empty string. A property such as org.eclipse.examples.MyBean.threadContext.unchanged= could be interpreted either way. In order for our users to have a solution, we can accommodate this by stating that both shall be interpreted by MicroProfile Concurrency to have the same meaning of indicating no context types. Thus, the user will be able to rely on specifying the blank value regardless of which way MP Config supplies that value to the MP Concurrency implementation.