avcodec/bsf: improve the doxy for av_bsf_flush()

Mention an example scenario where the function should be used.
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/bsf.h

@@ -215,7 +215,7 @@ int av_bsf_send_packet(AVBSFContext *ctx, AVPacket *pkt);
 int av_bsf_receive_packet(AVBSFContext *ctx, AVPacket *pkt);
 
 /**
- * Reset the internal bitstream filter state / flush internal buffers.
+ * Reset the internal bitstream filter state. Should be called e.g. when seeking.
  */
 void av_bsf_flush(AVBSFContext *ctx);