add callout for creating users

Author: josefaidt

src/pages/[platform]/build-a-backend/auth/use-existing-cognito-resources/index.mdx

@@ -128,7 +128,7 @@ export const auth = referenceAuth({
 
 <Callout info>
 
-Referenced resources cannot be modified. IAM policies specific to your Amplify application will be appended to your authenticated and unauthenticated roles.
+Referenced resources cannot be modified. IAM policies specific to your Amplify application will be appended to your authenticated and unauthenticated roles, and applications using the referenced resource will be able to create users in the Cognito user pool and identities in the Cognito identity pool.
 
 </Callout>