avformat/dashenc: Added comments

Added comments regarding usage of certain movflags in streaming mode.

libavformat/dashenc.c

@@ -1216,6 +1216,9 @@ static int dash_init(AVFormatContext *s)
 
         if (os->segment_type == SEGMENT_TYPE_MP4) {
             if (c->streaming)
+                // frag_every_frame : Allows lower latency streaming
+                // skip_sidx : Reduce bitrate overhead
+                // skip_trailer : Avoids growing memory usage with time
                 av_dict_set(&opts, "movflags", "frag_every_frame+dash+delay_moov+skip_sidx+skip_trailer", 0);
             else
                 av_dict_set(&opts, "movflags", "frag_custom+dash+delay_moov", 0);