State the implementation of HTML5 export for `user://` path in Data Paths (#5418)

tutorials/io/data_paths.rst

@@ -42,6 +42,10 @@ Linux, ``~/Library/Application Support/Godot/app_userdata/Name`` on macOS (since
 name defined in the Project Settings, but you can override it on a per-platform
 basis using :ref:`feature tags <doc_feature_tags>`.
 
+On HTML5 exports, ``user://`` will refer to a virtual filesystem stored on the
+device via IndexedDB. (Interaction with the main filesystem can still be performed
+through the :ref:`JavaScript <class_JavaScript>` singleton.)
+
 Converting paths to absolute paths or "local" paths
 ---------------------------------------------------