Document `Resource.duplicate()` only copying exported variables' values

(cherry picked from commit 2ba893e3ce80a92ac737c51a3fa9c840bdc5ca8a)

doc/classes/Resource.xml

@@ -27,6 +27,7 @@
 			<description>
 				Duplicates the resource, returning a new resource. By default, sub-resources are shared between resource copies for efficiency. This can be changed by passing [code]true[/code] to the [code]subresources[/code] argument which will copy the subresources.
 				[b]Note:[/b] If [code]subresources[/code] is [code]true[/code], this method will only perform a shallow copy. Nested resources within subresources will not be duplicated and will still be shared.
+				[b]Note:[/b] When duplicating a resource, only [code]export[/code]ed properties are copied. Other properties will be set to their default value in the new resource.
 			</description>
 		</method>
 		<method name="emit_changed">