I see design definitions in Custom controls as the name states are for providing a design description.
I see this more like a beautification part to your design. Cos I think , simple multiline comments also would serve the same purpose. The top side is , I looks good when included on the XPages.
The down side is, I feel little irrated to work with the xml mark up I have to provide in the Design Definitions tab.
Coming to the point, the simplest syntax that I learn is, you got to provide xml markup inside the Design Definitions text area as you would do in a normal XPage. Take a look at the following screen shot on how/where to provide the markup value
If you do this, you custom control when included on an XPage, would look something like the following on the designer
Looks pretty design to me :), at least better when compared to the native appearence