updated plotting documentation to describe color specification and masking
updated plotting documentation to describe color specification and masking

walshjon/openmc,smharper/openmc,bhermanmit/cdash,walshjon/openmc,keadyk/openmc_mg_prepush,mit-crpg/openmc,sxds/opemmc,paulromano/openmc,sxds/opemmc,johnnyliu27/openmc,mjlong/openmc,smharper/openmc,lilulu/openmc,mjlong/openmc,bhermanmit/openmc,wbinventor/openmc,liangjg/openmc,wbinventor/openmc,lilulu/openmc,kellyrowland/openmc,nhorelik/openmc,samuelshaner/openmc,smharper/openmc,smharper/openmc,amandalund/openmc,shenqicang/openmc,shikhar413/openmc,paulromano/openmc,keadyk/openmc_mg_prepush,amandalund/openmc,shikhar413/openmc,paulromano/openmc,johnnyliu27/openmc,amandalund/openmc,walshjon/openmc,johnnyliu27/openmc,mit-crpg/openmc,walshjon/openmc,shikhar413/openmc,samuelshaner/openmc,mit-crpg/openmc,wbinventor/openmc,johnnyliu27/openmc,liangjg/openmc,nhorelik/openmc,kellyrowland/openmc,samuelshaner/openmc,samuelshaner/openmc,paulromano/openmc,shenqicang/openmc,bhermanmit/openmc,keadyk/openmc_mg_prepush,amandalund/openmc,lilulu/openmc,wbinventor/openmc,mit-crpg/openmc,liangjg/openmc,shikhar413/openmc,liangjg/openmc