A warning line should look like a warning section in Guides

This is another improvement for Guides. related to #15823 and #15808.
What I can think of is getting a plain text page like this:
![screen shot 2014-06-19 at 9 58 09 pm](https://cloud.githubusercontent.com/assets/386234/3335890/7ca11bac-f81e-11e3-8cfe-bd0bc0f2bb02.png)