docs: dt: writing-schema: Document preferred order of properties

Document established Devicetree bindings maintainers review practice:
using DTS coding style property order in both 'properties' and
'required' secions.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250713-dt-bindings-docs-v2-4-672c898054ae@linaro.org


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