swresample: set flags & description and add documentation for output_sample_bits

Allows users to actually use such option.

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/resampler.texi

@@ -217,6 +217,10 @@ select Kaiser Windowed Sinc
 For swr only, set Kaiser Window Beta value. Must be an integer in the
 interval [2,16], default value is 9.
 
+@item output_sample_bits
+For swr only, set number of used output sample bits for dithering. Must be an integer in the
+interval [0,64], default value is 0, which means it's not used.
+
 @end table
 
 @c man end RESAMPLER OPTIONS

libswresample/swresample.c

@@ -127,7 +127,7 @@ static const AVOption options[]={
 
 { "kaiser_beta"         , "set swr Kaiser Window Beta"  , OFFSET(kaiser_beta)    , AV_OPT_TYPE_INT  , {.i64=9                     }, 2      , 16        , PARAM },
 
-{ "output_sample_bits"   , ""  , OFFSET(dither.output_sample_bits)               , AV_OPT_TYPE_INT  , {.i64=0                     }, 0      , 64        , 0 },
+{ "output_sample_bits"  , "set swr number of output sample bits", OFFSET(dither.output_sample_bits), AV_OPT_TYPE_INT  , {.i64=0   }, 0      , 64        , PARAM },
 {0}
 };