STYLE: update Model to Label Map xml file docs

More testing showed that the output volume will always be cast
to unsigned char, so limit the label value to 0-255 and
give the user some hints about good models to use.

Issue #3233



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22218 3bd1e089-480b-0410-8dfb-8563597acbee