add ASS encoding and decoding support in Subtitle Formats documentation

Originally committed as revision 26120 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/general.texi

@@ -690,7 +690,7 @@ performance on systems without hardware floating point support).
 
 @multitable @columnfractions .4 .1 .1 .1 .1
 @item Name @tab Muxing @tab Demuxing @tab Encoding @tab Decoding
-@item SSA/ASS      @tab X @tab X
+@item SSA/ASS      @tab X @tab X @tab X @tab X
 @item DVB          @tab X @tab X @tab X @tab X
 @item DVD          @tab X @tab X @tab X @tab X
 @item PGS          @tab   @tab   @tab   @tab X