Updated documentation for sitkShow environment variables.
Updated documentation for sitkShow environment variables.

I've augmented the Doxygen documentation for the sitkShow class to describe
the tokens allowed in the SITK_SHOW_COMMAND, SITK_SHOW_COLOR_COMMAND and
SITK_SHOW_3D_COMMAND environment variables.

Change-Id: Ice13ae953e04c14aadc70c23422cf22e26e6820d

SimpleITK/SimpleITK,richardbeare/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,hendradarwin/SimpleITK,blowekamp/SimpleITK,richardbeare/SimpleITK,kaspermarstal/SimpleElastix,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,hendradarwin/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,hendradarwin/SimpleITK,kaspermarstal/SimpleElastix,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,hendradarwin/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,hendradarwin/SimpleITK,blowekamp/SimpleITK,richardbeare/SimpleITK,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,richardbeare/SimpleITK,kaspermarstal/SimpleElastix,hendradarwin/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,hendradarwin/SimpleITK,kaspermarstal/SimpleElastix,blowekamp/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,hendradarwin/SimpleITK,InsightSoftwareConsortium/SimpleITK