doc pgm

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@41065 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/cairodriver/cairodriver.html

@@ -52,7 +52,7 @@ Several environment variables affect the operation of the Cairo driver:
        <LI><B>.png</B> - Portable Network Graphics (PNG)</LI>
        <LI><B>.bmp</B> - Windows Bitmap (BMP, 32-bpp)
            (these are not readable by some older viewers)</LI>
-       <LI><B>.ppm</B> - Portable Pixmap (PPM, PGM)</LI>
+       <LI><B>.ppm</B> - Portable Pixmap (PPM + PGM for alpha channel)</LI>
      </UL>
      Supported vector formats:
      <UL>

lib/pngdriver/pngdriver.html

@@ -33,7 +33,7 @@ of the PNG driver.
   <LI><B>GRASS_PNGFILE=filename</B><BR>
      the filename to put the resulting image in, default is <tt>map.png</tt>.
      If you set GRASS_PNGFILE to a filename which ends in ".ppm", a PPM 
-     file will be created. <!-- how about PGM? -->
+     file will be created (with alpha channel stored in a PGM image, if applicable).
      If you set GRASS_PNGFILE to a filename which ends in ".bmp", a 32-bpp
      BMP file will be created (these are not readable by some older viewers).
      <br><br>