Add clarying doc about node transformation additive vs. replacement
Add clarying doc about node transformation additive vs. replacement

CesiumGS/cesium,YonatanKra/cesium,likangning93/cesium,josh-bernstein/cesium,likangning93/cesium,CesiumGS/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,CesiumGS/cesium,YonatanKra/cesium,AnalyticalGraphicsInc/cesium,jasonbeverage/cesium,progsung/cesium,progsung/cesium,likangning93/cesium,YonatanKra/cesium,CesiumGS/cesium,josh-bernstein/cesium,likangning93/cesium,likangning93/cesium,jasonbeverage/cesium,YonatanKra/cesium