You may be able to do something with the instructions above. Let me know as we do this with our Raise User selection (employee id, username, etc) and our Urgency and Impact descriptions.
I'm not sure that the attribute template option is going to work in this scenario. Currently there isn't a way to apply a hint by hovering over a value.
You could create an attribute on your category object called Hint as a -1 field, and at the lowest level of the category, populate that value with your hint text. On your incident window, expand the category relationship and add your hint attribute to the window, underneath the category relationship for example.
Now when you select your category, the hint text will appear. The downside is, they would need to reselect the category.
This attribute doesn't need to be shown all the time, so it might be an idea to have two seperate incident windows - one for creation which will have the Hint attribute on it, and a second window for updating incidents without the hint attribute.
Hope that helps.