The documentation is being updated. There may be incomplete sections.
Working on a short tutorial on how to have rock formations alter the shape of dunes. Will publish it next change I get.
The basic idea is to mix two separate terrains (Dunes and Rock formation) with 100% Max mode in Combine, then use the Separation mask to drive the displacement of the dunes.
This tutorial will also cover an important concept I like to call "Intentional Orphans". This is where nodes are used for something other than their primary output.