“You Can’t Make This Stuff Up” is the theme of this week’s AFI Docs Film Festival in Washington, where some 70 documentaries will be shown in theatres across the city over five days.  To get myself in shape, I spent Sunday and Monday watching two documentaries that are not part of the festival but are currently playing in the area. One tried — and only partially succeeded — in reaching the standards suggested by the theme. The other is a masterpiece simply because it captures a treasure at the height of her powers.  As one reviewer phrased it, “She blew the doors off the joint.” But let’s start with the less-satisfying of the two. Echo in the Canyon, currently playing at the E Street Cinema, is a documentary about the legendary Laurel Canyon music scene in Los Angeles from the mid-1960s. The film focuses on the music of The Byrds, The Beach Boys, Buffalo Springfield, and The Mamas and the Papas, and the hook is a 2015 tribute concert from current-day fans Jakob Dylan (Bob’s …