Explain snapshot/rollback behavior better

Some SUSE Manager users reported they were confused about the snapshot/rollback UI, specifically when deciding which snapshot to use to revert a certain action (eg. assigning different channels to a host). We decided to reword the page slightly in order to make it clear that, when "reverting" a certain action, you should refer to the previous snapshot (rather than the snapshot that has that action itself in the description).

Before:
![before](https://cloud.githubusercontent.com/assets/250541/5166153/ddb05a34-73ed-11e4-8425-9f046c27d720.png)

After:
![after](https://cloud.githubusercontent.com/assets/250541/5166155/e281fdb0-73ed-11e4-8a19-80f88dbdcab9.png)