The filmmakers behind the new documentary “Super/Man: The Christopher Reeve Story” are preoccupied with the qualities that contribute to one being considered a hero. The basic story of the eponymous actor’s life lends itself to such an inquiry: Primarily known for playing Superman, one of the most Herculean and valiant comic book characters, Reeve became paralyzed at the age of 42 after a horseback riding incident in 1995. His later dedication to causes important to people with disabilities then turned him into another icon, one of fortitude and tenacity.