I attached the first version of the documentation for `run_coroutine_threadsafe`. The `Concurrency and multithreading` section also needs to be updated but I could already use some feedback.

Also, I think we should add a `try-except` in the callback function, especially since users can set their own task factory. For instance:
  
loop.set_task_factory(lambda loop, coro: i_raise_an_exception)

will cause the future returned by `run_coroutine_threadsafe` to wait forever. Instead, we could have:

    except Exception as exc:
        if future.set_running_or_notify_cancel():
            future.set_exception(exc)

inside the callback to notify the future.