Improve documentation of plgdidev, plgdiori, plgdiplt, plsdidev, plsdiori, and plsdiplt.  Used the &deg; symbol in plgdiori and plsdiori (for the first time in all the DocBook documentation), and it seemed to work well.
Improve documentation of plgdidev, plgdiori, plgdiplt, plsdidev, plsdiori,
and plsdiplt.  Used the &deg; symbol in plgdiori and plsdiori (for the first
time in all the DocBook documentation), and it seemed to work well.

svn path=/trunk/; revision=3527

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