Director Reginald Hudlin’s “Sidney” was made with the full and keenly interested cooperation of the Poitier family, following a template of access many documentaries favor or, in some cases, settle for. This is one of the good ones. Its format limitations (a stand-alone feature, under two hours) and the personnel involved lead, perhaps inevitably, to more of a classy overview of a singular breakthrough artist than a deeper examination of Poitier’s impact. Plainly, the man deserves something of the breadth of the six-hour HBO series on Paul Newman and Joanne Woodward. But with Poitier’s death e…