Explain default behavior of AssetServer in the asset_loading example (#…

…822)

Add clarification for where assets are loaded from in the`asset_loading`example

examples/asset/asset_loading.rs

@@ -15,7 +15,8 @@ fn setup(
     meshes: Res<Assets<Mesh>>,
     mut materials: ResMut<Assets<StandardMaterial>>,
 ) {
-    // You can load individual assets like this:
+    // By default AssetServer will load assets from inside the "assets" folder
+    // For example, the next line will load "assets/models/cube/cube.gltf#Mesh0/Primitive0"
     let cube_handle = asset_server.load("models/cube/cube.gltf#Mesh0/Primitive0");
     let sphere_handle = asset_server.load("models/sphere/sphere.gltf#Mesh0/Primitive0");