added advice how to reference existing lambda functions

src/pages/[platform]/build-a-backend/data/custom-business-logic/index.mdx

@@ -184,6 +184,8 @@ export const data = defineData({
 });
 ```
 
+If you want to use an existing lambda function, you can reference it by its name: `a.handler.function('name-of-existing-lambda-fn')`. This references an external lambda resource which Amplify is not aware of. You need to make sure the function and all dependencies are being managed outside of Amplify.
+
 </Block>
 <Block name="Custom resolver powered by AppSync JavaScript resolvers">