Merge pull request #353 from jonaspm/docs/auth-change-password

docs: Document changePassword method in auth module

docs/content/docs/(documentation)/modules/auth.mdx

@@ -30,6 +30,25 @@ With a focus on flexibility and ease of integration, bknd's authentication syste
 
 ---
 
+## Server Module: `module.auth`
+
+### `auth.changePassword([userId], [password])`
+
+To change a user's password, use the `changePassword` method:
+
+```ts
+await app.module.auth.changePassword(user.id, password);
+```
+
+This method updates the password for the specified user.
+- `userId`: The ID of the user whose password should be changed.
+- `password`: The new password value.
+
+This method throws an error if:
+- The user with the given `userId` is not found.
+- The user's authentication strategy is not `"password"` (e.g. the user registered via an OAuth provider).
+
+
 ## Securing Your Admin Portal
 
 <Callout type="warn" title="Do not enable the Guard without an admin user">