docs: dt: writing-schema: explain additional/unevaluatedProperties

Add to the list of schema contents expected keywords with their
explanation: additionalProperties and unevaluatedProperties.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240212150524.81819-2-krzysztof.kozlowski@linaro.org


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