BEHIND THE SEAMS: From Valentino to Anna Wintour, fashion documentaries have become the rage. Most recently, it was Rag & Bone’s turn to let the camera in on some fashion secrets. R.J. Cutler, of “September Issue” fame, filmed designers Marcus Wainwright and David Neville as they were preparing for their fall 2010 show. The short documentary, which was sponsored by Starbucks, will premiere on Sept. 8 with a screening at the Crosby Hotel cinema room, which will be followed with a Q & A session with Cutler. After the screening, it will be available for viewing at rag-bone.com and Starbucks’ frappuccino.com Web site. And in the buildup to the shows, who doesn’t need caffeine?