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>

Author: krzk

Documentation/devicetree/bindings/writing-bindings.rst

@@ -40,9 +40,9 @@ Properties
 ==========
 
 - DO make 'compatible' properties specific. DON'T use wildcards in compatible
-  strings. DO use fallback compatibles when devices are the same as or a subset
-  of prior implementations. DO add new compatibles in case there are new
-  features or bugs.
+  strings. DO use fallback compatibles when devices are the same as or a
+  superset of prior implementations. DO add new compatibles in case there are
+  new features or bugs.
 
 - DO use a vendor prefix on device-specific property names. Consider if
   properties could be common among devices of the same class. Check other