One of the classical music world’s leading virtuosos receives a cinematic hug with the documentary “Itzhak” (2017). Billed as “a revealing scrapbook of his favorite stories, “Itzhak” moves through vignettes from his life, including his polio diagnosis at age 4, his breakthrough victory at the Leventritt Competition in 1964 and his five-decade marriage to Toby Friedlander, herself an accomplished violinist.