Update plscompression documentation to describe those devices that use  the compression parameter and what it means.
Update plscompression documentation to describe those devices that use 
the compression parameter and what it means.


svn path=/trunk/; revision=9204

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot