There's no specific documentation besides looking at the source code of the configuration slicing plugin itself.
I had originally hoped that other plugin makers might adopt slicing implementations within their plugins, but that seems unlikely given the complexity of stacked plugin dependencies. The alternative would be to move the slicing functionality into Jenkins core, so other plugin implementors would be more likely to see it. A number of other contributors have picked up work on the slicing plugin in the last few years. As I look at the UI of slices, it seems a little overwhelming right now. So I think taming the UI complexity of selecting a slice would be the next big step for this plugin.
Perhaps it could be moved closer to the configuration page for individual projects, instead of listing everything at once.