Apply a warning icon to all warning texts in devel-docs

This pull req adds a simple consistent icon style to warning texts, to make them easier to notice, so each visitor can pay extra attention to information they shoud be aware of.

![capture du 2014-06-10 15 13 06](https://cloud.githubusercontent.com/assets/3578089/3240042/cd8f86ac-f11b-11e3-8a10-b4424cc98d42.png)