Patch by Anton Khirnov, wyskas at gmail
Originally committed as revision 23468 to svn://svn.ffmpeg.org/ffmpeg/trunk
/**
* Sets the given tag in m, overwriting an existing tag.
* @param key tag key to add to m (will be av_strduped depending on flags)
- * @param value tag value to add to m (will be av_strduped depending on flags)
+ * @param value tag value to add to m (will be av_strduped depending on flags).
+ * Passing a NULL value will cause an existing tag to be deleted.
* @return >= 0 on success otherwise an error code <0
*/
int av_metadata_set2(AVMetadata **pm, const char *key, const char *value, int flags);