documentation issue with --dump-json and --output

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The docs state:

-j, --dump-json                  Simulate, quiet but print JSON information.
                                 See --output for a description of available
                                 keys.

But --output sets the filename, and winds up altering the json to include

... "Cookie": "hdnea2=expires=1496978694~access=/adultswim/big/secure/tve/2017-05/ASOE1004271700075224_HLS/*~md5=179bc7398ac49178273600f5c3088d9d"}, "fulltitle": "Dragon Ball Z Kai - DBZ Kai 118", "_filename": "test.json"}

Note that "--output test.json" did not save the json, nor was there any docs for the available keys.