dts: bindings: improve documentation for Nordic IPs

Add some details to help new users learn how to manage the nodes
for compatibles corresponding to IP blocks on Nordic SoCs.

The focus is on IP blocks where the pinmux is configured in
devicetree. These typically have individual properties for each pin,
like a SCK pin on a SPI peripheral, which correspond directly to a
PSEL register value. These violate the usual devicetree convention of
using a 'foo-gpios <&gpioX ...>' phandle property and work in ways
that only make sense if you understand the underlying register map, so
it's worth explaining them a bit more carefully.

Skip these two bindings, which have no drivers: nordic,nrf-i2s.yaml,
nordic,nrf-pdm.yaml.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>