Actors — talented actors, that is — are usually amusing raconteurs and often good writers. They have a love of language, the sensuous savoring of the perfect phrase, and they develop a feeling for the dramatic arc of a scene, the timing of a punch line. So it is not surprising that Hal Holbrook, in his autobiography, “Harold: The Boy Who Became Mark Twain,” relates an arresting and vividly told life story, both personal and professional.