document about is_live flag

[AndCycle]

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as I looking through about how to filter live stream to avoid partial download by youtube-dl,
the result is only pop up here on github #12946

I think is_live flag is worth to mention in document to avoid any further question about it on issue list,

although I can checkout the source then do a pull req. but is_live is a boolean, which make me hesitate about where and how should I properly write the description, so probably someone else can make a better job on this as I am not native english speaker.