fix: protect demo users from deletion and password changes (#6819)

apps/docs/docs/management/users/index.mdx

@@ -18,6 +18,9 @@ password. Decimal units are the default. Change them from the user form or **Com
 Preferences → Data size units**. Byte rates follow the same preference; bit rates stay decimal and use labels such as
 Mb/s. Deleting a user does not delete boards they created.
 
+In demo mode (`DEMO_MODE=true`), user deletion and password changes are disabled for everyone, including administrators,
+even when `DEMO_READ_ONLY=false` allows other changes. This applies to the UI and API.
+
 ![The Users management page](../img/users.png)
 
 ## Groups