DOC: Adding intended use for each pixel list.
DOC: Adding intended use for each pixel list.

The pixel lists were intended to be used with specific image types.
Some of them are for itk::Image, others for itk::VectorImage, and
some are for itk::LabelMap. Comments were added make that intent
explicit.

SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging