docs: clarify cfnInclude parameters (#21001)

fixes #20905

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packages/@aws-cdk/cloudformation-include/lib/cfn-include.ts

@@ -50,7 +50,11 @@ export interface CfnIncludeProps {
    * If you include a parameter here with an ID that isn't in the template,
    * template creation will fail and an error will be thrown.
    *
-   * @default - no parameters will be replaced
+   * If you are importing a parameter from a live stack, we cannot know the value of that
+   * parameter. You will need to supply a value for your parameters, else the default
+   * value will be used.
+   *
+   * @default - parameters will retain their original definitions
    */
   readonly parameters?: { [parameterName: string]: any };
 }