Update docs for extended results

[johnarnold]

From #4490

Update http://docs.celeryproject.org/en/latest/userguide/calling.html#results-options with a description of the use case for it.

[huangsam]

Looking at this test case, it seems like result_extended is meant to be used for app configuration. It doesn't seem like a parameter you would pass into a call to apply_async. Does that mean the output from apply_async changes depending on whether that configuration is set or not? If so, how does the output change?

[thedrow]

We should test if result_extended works on tasks. I think it should.
Also, we added #5017 which could be used by users.

[thedrow]

I was wrong about what #5017 was all about.