swresample: add SwrDitherType doxy

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/swresample.h

@@ -113,6 +113,7 @@
 //TODO use int resample ?
 //long term TODO can we enable this dynamically?
 
+/** Dithering algorithms */
 enum SwrDitherType {
     SWR_DITHER_NONE = 0,
     SWR_DITHER_RECTANGULAR,