Add a quick description to the H.264 BSF

Originally committed as revision 18809 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264_mp4toannexb_bsf.c

 /*
+ * H.264 MP4 to Annex B byte stream format filter
  * Copyright (c) 2007 Benoit Fouet <benoit.fouet@free.fr>
  *
  * This file is part of FFmpeg.