Stanley Nelson has to admit that his timing was good. His new documentary, “The Black Panthers: Vanguard of the Revolution,” is starting to play at festivals — it had its premiere at Sundance last month — in the wake of the nationwide protests over police killings of African-American men in Ferguson, Mo., and Staten Island. The moment is right for a documentary history of the Panthers, with its images of armed black men patrolling the streets and walking onto the floor of the California Legislature.