docs: dt: writing-schema: document expectations on example DTS

Devicetree binding maintainers expect the example DTS in a binding to
show the usage of only this one particular binding, without unrelated
device nodes.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240212150524.81819-3-krzysztof.kozlowski@linaro.org


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