Singer recorded acoustic version of Nirvana classic at home on four-track recorder in 1988
Untitled song culls from cassette likely recorded around making of 'Nevermind'
"I didn't realize it was such a historical thing," 19-year-old Maggie Poukkula says. "I thought they were just cool pictures of my dad and Kurt jamming together"
"The dark stuff at the end I think would bum me out," singer says
Kurt Cobain 'was a master of jerking your chain,' explains Buzz Osborne.
HBO offers in-depth look at the upcoming documentary with never-before-seen home movies and family photos
Docs on Nina Simone and Scientology are also slated for the 2015 festival, along with new films staring James Franco, Jason Segel and Ewan McGregor