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

Documentation/devicetree/bindings/writing-schema.rst

@@ -92,8 +92,10 @@ additionalProperties / unevaluatedProperties
       'unevaluatedProperties: false'.  Typically bus or common-part schemas.
 
 examples
-  Optional. A list of one or more DTS hunks implementing the
-  binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.
+  Optional. A list of one or more DTS hunks implementing this binding only.
+  Example should not contain unrelated device nodes, e.g. consumer nodes in a
+  provider binding, other nodes referenced by phandle.
+  Note: YAML doesn't allow leading tabs, so spaces must be used instead.
 
 Unless noted otherwise, all properties are required.