Add a quick description to the H.264 BSF

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@18809 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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.