Its been some years now. I think I arrived at the youtube video through a link here in Hacker news.

I did not found the original documentary, but found this playlist in youtube with a mini doc anda few Zanrock albuns.

https://youtu.be/czrPRJehmdA?si=OKbqUQZw1cjCeG6E

Thanks, it looks like some other people found perhaps a different one in the sibling comments. It's interesting it seems to be transmitted among the unfamiliar via youtube serendipity and existing Zamrock enjoyers sneaking it into TV shows.