[imp] Show and improve the precheck description

### Tracker
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33831&start=0

### How to test

1. install Joomla 3.3.0
2. see at the summary view the precheck
3. no description is shown
4. see no_description_pre_installation.jpg
5. apply patch
6. description is shown
7. see new_pre_installation.jpg

#### Old precheck
![no_description_pre_installation](https://cloud.githubusercontent.com/assets/2596554/3211251/5a46f674-ef0b-11e3-88e7-952c26473cbc.JPG)

#### New precheck
![new_pre_installation](https://cloud.githubusercontent.com/assets/2596554/3211250/5a387022-ef0b-11e3-8a72-24b01ae059d9.JPG)