docs: dt: writing-schema: clarify that schema should describe hardware

The 'title' and 'description' fields in Devicetree schema is supposed to
describe hardware, not the binding itself.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240212150524.81819-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>