It’s not easy being the comedy king. And now “ Saturday Night Live ” creator Lorne Michaels is finally ready to let the world see just how hard it is to captain a 50-year-old pop culture institution. Michaels, the guarded Canadian producer who launched the live late-night sketch comedy show in 1975, is the subject of a new documentary called “Lorne,” which Focus Features will release in theaters on Friday. (NBCUniversal is the parent company of Focus Features, NBC and NBC News.) But the film