doc: dts: simplify DT_INST documentation

Add some introductory text to these driver helpers which makes it more
clear how they are defined in terms of the generic APIs, which allows
removing some boilerplate from doxygen. This also gives us a chance to
call out the exceptional cases and add some more documentation to those.

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