The way the nRF21540 device is configured makes sense once you
understand it, but it's quite unusual: a single peripheral is
configured with two separate devicetree nodes linked by a phandle.
Since this risks entering "exploding head" territory for beginners, it
deserves a thorough example. Add one to the binding's description.
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
a930bebb42