[extractor/common] Document duration meta field for playlists

youtube_dl/extractor/common.py

@@ -336,8 +336,8 @@ class InfoExtractor(object):
     object, each element of which is a valid dictionary by this specification.
 
     Additionally, playlists can have "id", "title", "description", "uploader",
-    "uploader_id", "uploader_url" attributes with the same semantics as videos
-    (see above).
+    "uploader_id", "uploader_url", "duration" attributes with the same semantics
+    as videos (see above).
 
 
     _type "multi_video" indicates that there are multiple videos that