[ffmpeg] --add-metadata: Set comment and purl fields (Fixes #4847)

youtube_dl/postprocessor/ffmpeg.py

@@ -511,8 +511,9 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
             metadata['artist'] = info['uploader_id']
         if info.get('description') is not None:
             metadata['description'] = info['description']
+            metadata['comment'] = info['description']
         if info.get('webpage_url') is not None:
-            metadata['comment'] = info['webpage_url']
+            metadata['purl'] = info['webpage_url']
 
         if not metadata:
             self._downloader.to_screen('[ffmpeg] There isn\'t any metadata to add')