Add docs on tasks depending on other async tasks

I found myself getting confused when I had a task that depended on
another that was async. After a while I realised why and thought adding
a couple of sentences just to clarify for future users who may come
stuck with the same problem.

I dropped you guys a tweet earlier: https://twitter.com/Jack_Franklin/status/408017546894651392, before some help from Eric cleared things up - I'm not sure if you feel this is a problem many will come across but it had me befuddled for a few moments so I thought a quick 2 sentences in the docs might help others avoid the problem. I don't think it's immediately apparent why returning the stream works as a solution, so I hope this might clarify it.