dt-bindings: writing-schema: Explain sub-nodes with additionalProperties:true

Document recently introduced pattern of using additionalProperties: true
for sub-nodes with their own schema.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250331081345.37103-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/example-schema.yaml

@@ -178,7 +178,9 @@ properties:
     description: Child nodes are just another property from a json-schema
       perspective.
     type: object  # DT nodes are json objects
-    # Child nodes also need additionalProperties or unevaluatedProperties
+    # Child nodes also need additionalProperties or unevaluatedProperties, where
+    # 'false' should be used in most cases (see 'child-node-with-own-schema'
+    # below).
     additionalProperties: false
     properties:
       vendor,a-child-node-property:
@@ -189,6 +191,17 @@ properties:
     required:
       - vendor,a-child-node-property
 
+  child-node-with-own-schema:
+    description: |
+      Child node with their own compatible and device schema which ends in
+      'additionalProperties: false' or 'unevaluatedProperties: false' can
+      mention only the compatible and use here 'additionalProperties: true'.
+    type: object
+    additionalProperties: true
+    properties:
+      compatible:
+        const: vendor,sub-device
+
 # Describe the relationship between different properties
 dependencies:
   # 'vendor,bool-property' is only allowed when 'vendor,string-array-property'