Although Robert Downey Jr. is currently one of the most famous actors on the planet, he reminds the audience early in the new documentary “Sr.”— about his late father Robert Downey Sr.— that for many years, he was simply just known as “Bob Downey’s kid.” Which is ostensibly the impetus of “Sr.,” directed by celebrated documentarian Chris Smith (“American Movie”), yet, essentially hosted and presented by RDJ, to put his father in proper cinematic and historical context.