Document that slices can be bottom to top or top to bottom.

git-svn-id: file:///var/local/repositories/mplayer/trunk/libswscale@29665 b3059339-0415-0410-9bf9-f77b7e298cf2

swscale.h

@@ -135,7 +135,7 @@ struct SwsContext *sws_getContext(int srcW, int srcH, enum PixelFormat srcFormat
 /**
  * Scales the image slice in srcSlice and puts the resulting scaled
  * slice in the image in dst. A slice is a sequence of consecutive
- * rows in an image.
+ * rows in an image. Slices can be bottom to top or top to bottom.
  *
  * @param context   the scaling context previously created with
  *                  sws_getContext()