Fixes #15389, added docs to RawHandleWrapper::_window field (#18832)

# Objective

- Fixes https://github.com/bevyengine/bevy/issues/15389.
- Add documentation for RawHandleWrapper::_window field since It's
needed to drop the window at the correct time.

## Solution

- Added documentation to RawHandleWrapper::_window field as same as
WindowWrapper documentation.

## Testing

- No testing needed since it is documentation. 

---

crates/bevy_window/src/raw_handle.rs

@@ -50,6 +50,10 @@ impl<W: 'static> Deref for WindowWrapper<W> {
 /// thread-safe.
 #[derive(Debug, Clone, Component)]
 pub struct RawHandleWrapper {
+    /// A shared reference to the window.
+    /// This allows us to extend the lifetime of the window,
+    /// so it doesn’t get eagerly dropped while a pipelined
+    /// renderer still has frames in flight that need to draw to it.
     _window: Arc<dyn Any + Send + Sync>,
     /// Raw handle to a window.
     window_handle: RawWindowHandle,