dts: bindings: improve nordic,nrf21540-fem docs

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>