Doxygenate avcodec_version().

Originally committed as revision 14666 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -2528,8 +2528,11 @@ extern AVCodec *first_avcodec;
 #endif
 AVCodec *av_codec_next(AVCodec *c);
 
-/* returns LIBAVCODEC_VERSION_INT constant */
+/**
+ * Returns the LIBAVCODEC_VERSION_INT constant.
+ */
 unsigned avcodec_version(void);
+
 #if LIBAVCODEC_VERSION_INT < ((52<<16)+(0<<8)+0)
 /* returns LIBAVCODEC_BUILD constant */
 attribute_deprecated unsigned avcodec_build(void);