Cookbook: scope the metadata recipe to video, document what MeTube already sets
The Embed metadata recipe sets writethumbnail, which suppresses MeTube's
own audio thumbnail chain (ExtractAudio -> ThumbnailsConvertor -> Metadata
-> EmbedThumbnail): the guard tests whether the key is present rather than
its value, so 'true' disables the chain exactly as 'false' would. Following
the recipe on an audio download therefore produced worse results than no
configuration at all, and reintroduced the failure that #188 was fixed for
(issue #811 was closed against yt-dlp on that basis).
Adds a 'What MeTube already sets for you' section and a cli_to_api.py note
warning against pasting converter output wholesale, with the #589
square-cropped cover as the worked example.
Alex Shnitman