Improve documentation for IndependentComponents

Describe specific features for dependent component volumes.

Rendering/Core/vtkVolumeProperty.h

@@ -70,12 +70,17 @@ public:
   // determine functionality). If IndependentComponents is Off, then you
   // must have either 2 or 4 component data. For 2 component data, the
   // first is passed through the first color transfer function and the
-  // second component is passed through the first opacity transfer function.
-  // Normals will be generated off of the second component. For 4 component
+  // second component is passed through the first scalar opacity (and
+  // gradient opacity) transfer function.
+  // Normals will be generated off of the second component. When using gradient
+  // based opacity modulation, the gradients are computed off of the
+  // second component. For 4 component
   // data, the first three will directly represent RGB (no lookup table).
   // The fourth component will be passed through the first scalar opacity
-  // transfer function for opacity. Normals will be generated from the fourth
-  // component.
+  // transfer function for opacity and first gradient opacity transfer function
+  // for gradient based opacity modulation. Normals will be generated from the
+  // fourth component. When using gradient based opacity modulation, the
+  // gradients are computed off of the fourth component.
   vtkSetClampMacro(IndependentComponents, int, 0, 1);
   vtkGetMacro(IndependentComponents, int);
   vtkBooleanMacro(IndependentComponents, int);