Expand the description of vtkImageMapToWindowLevelColors

This filter is most often to apply a Window/Level operation to
grayscale input data, without a lookup table, so I have put that
first in the description.  The less-common use of modulating the
color produced by a lookup table has been moved to the end.

Imaging/Color/vtkImageMapToWindowLevelColors.h

@@ -14,16 +14,27 @@
 =========================================================================*/
 /**
  * @class   vtkImageMapToWindowLevelColors
- * @brief   map the input image through a lookup table and window / level it
- *
- * The vtkImageMapToWindowLevelColors filter will take an input image of any
- * valid scalar type, and map the first component of the image through a
- * lookup table.  This resulting color will be modulated with value obtained
- * by a window / level operation. The result is an image of type
- * VTK_UNSIGNED_CHAR. If the lookup table is not set, or is set to nullptr, then
- * the input data will be passed through if it is already of type
- * VTK_UNSIGNED_CHAR.
+ * @brief   Map an image through a lookup table and/or a window/level.
  *
+ * The vtkImageMapToWindowLevelColors filter can be used to perform
+ * the following operations depending on its settings:
+ * -# If no lookup table is provided, and if the input data has a single
+ *    component (any numerical scalar type is allowed), then the data is
+ *    mapped through the specified Window/Level.  The type of the output
+ *    scalars will be "unsigned char" with a range of (0,255).
+ * -# If no lookup table is provided, and if the input data is already
+ *    unsigned char, and if the Window/Level is set to 255.0/127.5, then
+ *    the input data will be passed directly to the output.
+ * -# If a lookup table is provided, then the first component of the
+ *    input data is mapped through the lookup table (using the Range of
+ *    the lookup table), and the resulting color is modulated according
+ *    to the Window/Level.  For example, if the input value is 500 and
+ *    the Window/Level are 2000/1000, the output value will be RGB*0.25
+ *    where RGB is the color assigned by the lookup table and 0.25 is
+ *    the modulation factor.
+ * See SetWindow() and SetLevel() for the equations used for modulation.
+ * To map scalars through a lookup table without modulating the resulting
+ * color, use vtkImageMapToColors instead of this filter.
  * @sa
  * vtkLookupTable vtkScalarsToColors
 */