Split PGM and PPM in image formats section.

patch by Daniel Verkamp, daniel drv nu

Originally committed as revision 17700 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/general.texi

@@ -245,11 +245,14 @@ following image formats are supported:
     @tab PAM is a PNM extension with alpha support.
 @item PCX          @tab   @tab X
     @tab PC Paintbrush
-@item PGM, PPM     @tab X @tab X
+@item PGM          @tab X @tab X
+    @tab Portable GrayMap image
 @item PGMYUV       @tab X @tab X
     @tab PGM with U and V components in YUV 4:2:0
 @item PNG          @tab X @tab X
     @tab 2/4 bpp not supported yet
+@item PPM          @tab X @tab X
+    @tab Portable PixelMap image
 @item PTX          @tab   @tab X
     @tab V.Flash PTX format
 @item RAS          @tab   @tab X