document tasks/coroutines properly

[StefanKarpinski]

The current documentation is severely lacking:

http://docs.julialang.org/en/latest/manual/control-flow/#man-tasks

This is probably one of the most poorly documented aspects of the language. Now that all of our I/O is task-based, this is a rather unfortunate oversight.

[JeffBezanson]

I think the docs for this are decent now.

[femtotrader]

I don't see in Task/Coroutine doc how to signal consumer that there is nothing to consume anymore.

Python generator raises exception (I don't remember exactly differences between StopIteration and GeneratorExit exceptions)

https://docs.python.org/3/library/exceptions.html#StopIteration
https://docs.python.org/3/library/exceptions.html#GeneratorExit

Is there a similar mechanism with Julia ?