Improved region boundary RAG docs

doc/examples/segmentation/plot_rag_boundary.py

@@ -4,7 +4,12 @@ Region Boundary based RAGs
 ==========================
 
 This example demonstrates construction of region boundary based RAGs with the
-`rag_boundary` function.
+:py:func:`skimage.future.graph.rag_boundary` function. The function taken an
+``edge_map`` argument, which gives the significance of a feature (such as
+edges) being present at each pixel. In a region boundary RAG, the edge between
+two regions is the average value of the corresponding pixels in ``edge_map``
+along their shared boundary.
+
 """
 from skimage.future import graph
 from skimage import data, segmentation, color, filters, io