STYLE: Remove wrong signed/unsigned warning to ThresholdScalarVolume

As of r10439, the CLI doesn't force the output to be signed.
Update the documentation.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21623 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Add Negate property to Treshold Scalar Volume CLI

As of today, there is no easy mechanism in ITK to do so.
See ITK issues:
https://issues.itk.org/jira/browse/ITK-2991
https://issues.itk.org/jira/browse/ITK-2993

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21624 3bd1e089-480b-0410-8dfb-8563597acbee