For what it’s worth, celebrities are often regarded as vain, narcissistic, self-absorbed and unapologetically materialistic, generally consumed with their own ambitions, being the center of attention and lining their pockets. Seldom are they widely recognized for qualities like artistic integrity, personal fulfillment and noble intentions. But those generalizations do not always provide realistic depictions; there are individuals who indeed embody sincere motivations that drive their craft and their lives. But how do they defy the so-called model and turn out like that? Such is the point behind the new Netflix documentary profile of actor-comedian Martin Short, “Marty, Life is Short.”