cmake: modules: dts: document outcome

When the build system was being split up into modules under
cmake/modules, most of the resulting cmake modules had their inputs
and outputs documented in top-of-file comments. The dts module is an
exception, which makes it harder to use since its contracts aren't
defined. Fix this by adding a contract.

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