Document register_avcodec().

Originally committed as revision 16132 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -2563,6 +2563,11 @@ unsigned avcodec_version(void);
  */
 void avcodec_init(void);
 
+/**
+ * Register the codec \p codec and initialize libavcodec.
+ *
+ * @see avcodec_init()
+ */
 void register_avcodec(AVCodec *codec);
 
 /**