Chris Willman Music WriterOn Tuesday, David Crosby declared to Howard Stern’s listeners that filmmaker Robert Zemeckis would be directing a documentary on Crosby, Stills & Nash, making possible use of “10,000 hours” of existing footage as well as fresh interviews. On Wednesday, that announcement got walked back a bit, with word that the tantalizing prospect of a full-on CSN doc is still only under discussion.“They’re gonna do a huge documentary on CSN.