docs: dt: writing-bindings: Rephrase typical fallback (superset) usage

docs: dt: writing-bindings: Rephrase typical fallback (superset) usage

When speaking about compatibles for new devices comparing to "prior
implementations", usually we expect new device to come with more
features, thus logically it is a superset, not subset, of "prior
implementations".

Suggested-by: Conor Dooley <conor@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250707095019.66792-4-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>