Document the %(format_id)s field for the output template

youtube_dl/__init__.py

@@ -336,7 +336,8 @@ def parseOpts(overrideArguments=None):
                   '%(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, '
-                  '%(format)s for the format description (like "22 - 1280x720" or "HD")'
+                  '%(format)s for the format description (like "22 - 1280x720" or "HD"),'
+                  '%(format_id)s for the unique id of the format (like Youtube\'s itags: "137"),'
                   '%(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, '