The director-producer behind the new “American Masters” documentary “Miles Davis: Birth of the Cool” discusses his non-profit Firelight Media venture, which offers a Documentary Lab program to help promising filmmakers develop projects. Nelson, a 2002 recipient of a MacArthur Foundation “genius” grant, explains how a community of documentarians has emerged from the lab program, and why he decided this year to add the William Greaves Fund program to support directors with more experience.