iOS App Extensions: add warning to mention that purchases aren't al…

…lowed

We had a customer recently try to purchase directly from a Safari extension. Hopefully calling out this limitation will save time for some people.

docs_source/🚀 Getting Started/configuring-sdk/ios-app-extensions.md

@@ -21,6 +21,12 @@ After you enable app groups, you will be able to access a user's active subscrip
 ]
 [/block]
 
+Now the app extension and parent app can both use the shared UserDefaults suite.
 
-
-Now the app extension and parent app can both use the a shared UserDefaults suite.
+[block:callout]
+{
+  "type": "warning",
+  "title": "Purchases aren't allowed on extensions",
+  "body": "Even though you can configure the SDK, it's only read-only. Purchasing will not work because extensions don't have access to the parent's app Bundle and therefore can't extract the receipt after a purchase."
+}
+[/block]