Add that webhook handlers can access user private metadata (#2739)

Author: jfoshee

docs/guides/users/extending.mdx

@@ -124,7 +124,7 @@ On the backend, it's available on the [Backend `User`](/docs/reference/backend/t
 
 ### Private metadata
 
-Private metadata is only accessible by the backend, which makes this useful for storing sensitive data that you don't want to expose to the frontend. For example, you could store a user's Stripe customer ID.
+Private metadata is only accessible by the backend and webhook handlers, which makes this useful for storing sensitive data that you don't want to expose to the frontend. For example, you could store a user's Stripe customer ID.
 
 #### Set private metadata