The smooth, purposeful voice that pushed “Ain’t No Sunshine” and “Lean on Me” up r&b and pop charts airs late-life-crisis grumbles and practiced reminiscences in a relaxed new documentary. Bill Withers, who nailed sounds-of-the-’70s hooks from touchy funk (“Use Me” and its wary-wicked clavinet) to adult-contempo bliss-out (“Lovely Day”), celebrates his 70th birthday here with the requisite musing about keeping a hand in recording. Though he’s ready with easy charm, his sensibly jaded edge makes things interesting (both tendencies may be reactions to stuttering since childhood). Thirteenth-born in a West Virginia mining family, and late to the business in his early thirties, Withers held on to a factory-line job at first, and maintains a mythos about scant performing experience despite what his Navy buddies say. He walks railroad ties in the old hometown, visits stuttering kids, amiably deflates a forced rap session on selling out with Cornel West and Tavis Smiley (Withers: “We’re all entrepreneurs”), and refrains from criticizing his aspiring-singer daughter. Happily married to an MBA, with a son headed to law school, Withers gets a sleepily even-keel portrait that could use more on musical technique, though it is nice to see him get happy with singer-songwriter Raul Midón.