doc: Clarify documentation for fade duration.

doc/filters.texi

@@ -4538,7 +4538,8 @@ The number of seconds for which the fade effect has to last. At the end of the
 fade-in effect the output video will have the same intensity as the input video,
 at the end of the fade-out transition the output video will be filled with the
 selected @option{color}.
-If both duration and nb_frames are specified, duration is used. Default is 0.
+If both duration and nb_frames are specified, duration is used. Default is 0
+(nb_frames is used by default).
 
 @item color, c
 Specify the color of the fade. Default is "black".