added advice how to reference existing lambda functions (#8136)

Co-authored-by: Thomas Reulen <[email protected]>

Author: ThomasReulen

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.
+
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