docs(ui): clarify UIElement value is read-only

marimo/_plugins/ui/_core/ui_element.py

@@ -101,7 +101,9 @@ class UIElement(Html, Generic[S, T]):
 
     **Attributes.**
 
-    - value: The value of the `UIElement`.
+    - value: The current value of the `UIElement`. Read-only; it reflects
+      frontend state and can't be assigned directly. If you need to
+      imperatively drive UI state, use `mo.state()`.
 
     **Methods.**
 
@@ -310,7 +312,12 @@ def _register_as_view(self, parent: UIElement[Any, Any], key: str) -> None:
 
     @property
     def value(self) -> T:
-        """The element's current value."""
+        """The element's current value.
+
+        Read-only; marimo updates it when the UI element changes in the
+        frontend. If you need to imperatively drive UI state, use
+        `mo.state()` instead of assigning to this property.
+        """
         if self._ctx is None:
             return self._value