[Doc] Improve ra-realtime doc to unlock record

Author: erwanMarmelab

docs/useLockOnMount.md

@@ -58,6 +58,8 @@ const PostEdit = () => (
 ```
 {% endraw %}
 
+**Note**: If users close their tab/browser when on a page with a locked record, `useLockOnMount` will call your `dataProvider.unlock` method. However, as browsers don't wait for asynchronous calls to finish, make sure your dataProvider executes as fast as possible.
+
 ## Parameters
 
 `useLockOnMount` accepts a single options parameter, with the following properties (all optional):