Have descriptions for each role on the selection page

For a site I am working on, I wanted to have a description for each of the roles on the select page e.g. what their different permissions where, cost, etc. I figured it wouldn't be too hard to have this functionality, so I managed to code it myself. If anyone else feels they want this functionality, here's the patch.

The patch provides an admin/settings page where you can enter a description for whatever roles you want. That description is then displayed below the role name on the role selection page. You can leave the description textarea blank if you don't want descriptions to appear.