Even though I hate the way that Robertson and Scorcese edited the film to try and make Robertson look like the creative genius and the rest look like drugged-out flunkies, still the best of its kind. I didn't get in to documentaries here because I thought the OP excluded them, but yeah there's no topping that one for music documentaries imo.
Although much less heralded and known, Festival Express is another great music documentary from earlier in the 70s, featuring a train tour across Canada with The Band, Grateful Dead, Janis, Buddy Guy, Flying Burrito Brothers and Delaney & Bonnie & Friends. Some epic scenes and performances, and the promoter is a very compelling character. By way of foreshadowing some of the problems that would ultimately drive The Band apart, it's notable that Robertson refused to take the train with the rest because he didn't want to party, so he drove and met them at stops along the way and is barely in the movie as a result. He sure missed a lot of fun, as the drunken scenes on the train are some of the best parts (remarkably, Jerry appears to be the most lucid one of the bunch).
And a much, much less heralded and lesser known documentary I just saw recently is the excellent Bayou Maharajah, about James Booker, the legendary New Orleans piano player. Can't recommend this one enough. Tragic and compelling story about a real genius and one of the most original and talented musicians, ever. It's on Netflix and Hulu, etc.