Document %(format)s (#1612)

youtube_dl/__init__.py

@@ -332,7 +332,9 @@ def parseOpts(overrideArguments=None):
             help=('output filename template. Use %(title)s to get the title, '
                   '%(uploader)s for the uploader name, %(uploader_id)s for the uploader nickname if different, '
                   '%(autonumber)s to get an automatically incremented number, '
-                  '%(ext)s for the filename extension, %(upload_date)s for the upload date (YYYYMMDD), '
+                  '%(ext)s for the filename extension, '
+                  '%(format)s for the format description (like "22 - 1280x720" or "HD")'
+                  '%(upload_date)s for the upload date (YYYYMMDD), '
                   '%(extractor)s for the provider (youtube, metacafe, etc), '
                   '%(id)s for the video id , %(playlist)s for the playlist the video is in, '
                   '%(playlist_index)s for the position in the playlist and %% for a literal percent. '