Add a comment about excluding skirts from OBBs

Source/Core/CesiumTerrainProvider.js

@@ -428,6 +428,12 @@ define([
         if (rectangle.width < CesiumMath.PI_OVER_TWO + CesiumMath.EPSILON5) {
             // Here, rectangle.width < pi/2, and rectangle.height < pi
             // (though it would still work with rectangle.width up to pi)
+
+            // The skirt is not included in the OBB computation. If this ever
+            // causes any rendering artifacts (cracks), they are expected to be
+            // minor and in the corners of the screen. It's possible that this
+            // might need to be changed - just change to `minimumHeight - skirtHeight`
+            // A similar change might also be needed in `upsampleQuantizedTerrainMesh.js`.
             orientedBoundingBox = OrientedBoundingBox.fromRectangle(rectangle, minimumHeight, maximumHeight, provider._tilingScheme.ellipsoid);
         }