update discussion blackout setting warning (TNL-442)

@mlkwaqas @lamagnifica (thanks for the copy!)

here's the updated doc in https://openedx.atlassian.net/browse/TNL-442

here's a screenshot of what the new wording will look like: 
![screen shot 2014-09-29 at 1 59 59 pm](https://cloud.githubusercontent.com/assets/2748698/4446459/8e7c2b80-4802-11e4-9a79-cf6429c1d483.png)