change text for case list icon feature preivew and add link

response to https://github.com/dimagi/commcare-hq/pull/3462

![screen shot 2014-05-19 at 4 42 00 pm](https://cloud.githubusercontent.com/assets/137212/3019756/2c39aa08-df96-11e3-9395-eb2aeb716b36.png)