1. Add a surface layer. Name it "Stones distribution" or anything you want to control
2. Uncheck the "Apply color" box.
3. Set the slope constraints to limit the maximum slope however you want it.
4. Click on the "Add Child Layer" button to add a child to the "distribution" layer. Select Fake stones shader under the displacement shader menu.
5. Adjust the fakestone size, colour, density, etc. however you need them.
This method is also good for when you want a rocky shoreline. Set the altitude constraints to limit the height to about 10-20m above the water line.
You can also distribute objects such as trees and grasses the same way.
The only trouble I can see with this method is that any colour you apply to the fakestones is distributed with the same rules as the underlying layer, so basically you only get colour on top of the stones and not on the sides. Anyone know a way around this?