In 20,000 Days on Earth, Nick Cave, one of the most dynamic and inventive musicians that Rock & Roll has ever produced, gets an innovative, fascinating and unconventional “documentary” worthy of its subject. The word is in quotations because filmmakers Iain Forsyth and Jane Pollard use Cave as a sort of untrustworthy narrator of his legacy: Does he really have his own museum archives? But he’s charming, articulate and inspiring about his need to create his artfully bluesy, soulful music, and his performances are superb.