HDRenderPipeline: Rename Use Opacity as density

ScriptableRenderPipeline/HDRenderPipeline/Material/LayeredLit/Editor/LayeredLitUI.cs

             public readonly GUIContent useHeightBasedBlendText = new GUIContent("Use Height Based Blend", "Layer will be blended with the underlying layer based on the height.");
             public readonly GUIContent useMainLayerInfluenceModeText = new GUIContent("Main Layer Influence", "Switch between regular layers mode and base/layers mode");
 
-            public readonly GUIContent opacityAsDensityText = new GUIContent("Use Opacity as Density", "Use Opacity as Density.");
+            public readonly GUIContent opacityAsDensityText = new GUIContent("Opacity map use as Density map", "Use opacity map as (alpha channel of base color) as Density map.");
             public readonly GUIContent inheritBaseNormalText = new GUIContent("Normal influence", "Inherit the normal from the base layer.");
             public readonly GUIContent inheritBaseHeightText = new GUIContent("Heightmap influence", "Inherit the height from the base layer.");
             public readonly GUIContent inheritBaseColorText = new GUIContent("BaseColor influence", "Inherit the base color from the base layer.");