doc: dts: clarify a design requirement

There's been some confusion about what we mean by hardware
description. We're really talking about the user-facing inputs that
need to be configured per SoC, application, etc. It's fine to do
things like use ifdeffery on a CMSIS header in an aarch32 support file
to decide if the current target has some feature, for instance -- that
sort of thing doesn't *have* to come from DT.

At the same time, we don't want to encourage vendor-specific hardware
configuration languages from creeping into upstream zephyr, so keep
the language strong in an effort to avoid that misinterpretation.

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