The artist C. Tangana, whose real name is Anton Alvarez, has set aside his mic and taken up directing to critical acclaim. His first feature-length documentary, “The Flamenco Guitar of Yerai Cortés,” has been nominated for Spain’s top film award and is winning  praise at festivals. Alvarez’s film goes beyond the music to explore family — and one family secret in particular. The World’s Gerry Hadden reports from Barcelona.