doc: dts: document zephyr, prefix for props/compats

We have established a convention that the zephyr, prefix in property
names indicates some sort of zephyr-specific extension to a common
binding, or a zephyr-specific driver configuration knob. We also have
established a convention that compatibles which begin with "zephyr,"
are specific to our operating system. Document these facts.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>