Updated docs for bevy::render::RenderSystems (#21586)

# Objective

- Fixes #16654 

## Solution

- Updated the doc string and linked to TemporaryRenderEntity.

## Testing

- Built the documentation locally and verified that the links render and
navigate correctly.

crates/bevy_render/src/lib.rs

@@ -186,7 +186,8 @@ pub enum RenderSystems {
     Render,
     /// Cleanup render resources here.
     Cleanup,
-    /// Final cleanup occurs: all entities will be despawned.
+    /// Final cleanup occurs: any entities with
+    /// [`TemporaryRenderEntity`](sync_world::TemporaryRenderEntity) will be despawned.
     ///
     /// Runs after [`Cleanup`](RenderSystems::Cleanup).
     PostCleanup,