Document event_loop_factory usage

Add a couple of lines mentioning event_loop_factory - no clear error message is given if you attempt to perform I/O in tasks created in this fashion. I spent a many hours debugging this yesterday which would have been avoided if it were documented.
The test case is also auto-wrapped in `fn main()` where `extern crate` definitions aren't allowed, so you'll have to add a secret `fn main()` wrapper yourself. I'd recommend running `rustdoc --test` just to make sure it checks out.