Marcin,
Unfortunately, you'd have to create a custom DataFactory for that. The main issue is even if there was a CategoryDescriptionAttribute (or CategoryAttribute accepted a description), you could have conflicting descriptions and you've have to set it for every property.
You would need to create a custom DataFactory (or one that derives from one of our existing factories) in order to set a description. Effectively, you'd create an object that extends CategoryDataAccessor and use that (with an appropriate description). Currently, you'd have to duplicate alot of logic built into the DataFactory to use your custom CategoryDataAccessor, while keeping the same functionality. For the 2010.1 release, I made a minor change to allow you to easily insert your custom CategoryDataAccessor.
When that is out, you'd create a class that derives from TypeDescriptorFactory (or TypeReflectionFactory) and then override the CreateCategoryDataAccessor method. In there you'd create a new instance of your custom CategoryDataAccessor.
