WASHINGTON — A little-known element of President Joe Biden’s massive stimulus relief package passed by the Senate on Saturday would pay billions of dollars to disadvantaged farmers — a provision that would benefit Black farmers in a way that some experts say no legislation has since the Civil Rights Act of 1964.Of the $10.4 billion in the American Rescue Plan that would support agriculture, about half would go to disadvantaged farmers, according to estimates from the Farm Bureau, an industry organization.