You are right that this feature is not documented yet, and we'll make sure to add it shortly.
"Support for sprites" means that there is a new property EnableImageSprites for the whole toolbar and EnableImageSprite for every toolbar button - replaces the toolbar image elements with a sized span for easier use of sprites. Setting ImageUrl overrides this behavior, so it should be removed for EnableImageSprite/s to work. These properties are also added for RadMenu.
the Telerik team