docs: Add callbacks example for Credential Provider

docs/docs/guides/providers/credentials-provider.md

@@ -69,7 +69,18 @@ providers: [
 ...
 ```
 
-See the [callbacks documentation](/reference/configuration/auth-config#callbacks) for more information on how to interact with the token.
+See the [callbacks documentation](/reference/configuration/auth-config#callbacks) for more information on how to interact with the token. For example, you can add additional information to the token by returning an object from the `jwt()` callback:
+  
+```js
+callbacks: {
+  async jwt(token, user, account, profile, isNewUser) {
+    if (user) {
+      token.id = user.id
+    }
+    return token
+  }
+}
+```
 
 ## Example - Web3 / Signin With Ethereum
 

packages/core/src/providers/credentials.ts

@@ -77,6 +77,19 @@ export type CredentialsProviderType = "Credentials"
  * with supporting usernames and passwords.
  *
  * :::
+ * 
+ * See the [callbacks documentation](/reference/configuration/auth-config#callbacks) for more information on how to interact with the token. For example, you can add additional information to the token by returning an object from the `jwt()` callback:
+ *   
+ * ```js
+ * callbacks: {
+ *   async jwt(token, user, account, profile, isNewUser) {
+ *     if (user) {
+ *       token.id = user.id
+ *     }
+ *     return token
+ *   }
+ * }
+ * ```
  *
  * @example
  * ```js