The Frontline documentary “America After 9/11” paints a justifiably grim picture of the failures over the past 20 years of American policies and actions in Afghanistan and Iraq. The series is produced in Boston, which played an outsized role in the 9/11 attacks, due to lax airport security. The documentary doesn’t mention that, nor does it mention the Boston Marathon bombings in 2013 by the Tsarnaev brothers.