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.
auto merge of #15576 : mrmonday/rust/patch-1, r=alexcrichton

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.