It is not possible to move the description above the field if you are using the out-of-the-box layout types. You need to create a custom layout manager class where you can specify the layout of labels, controls, and descriptions for a field in any way you want.
You can find details on how to implement this under "Custom Layout Help" section of the Help file, and also use the sample code (pleLayoutManager & pleCustomActionsAndCustomLayouts classes) shipped with the product as a starting point.
Thanks,
Yuan