Document setting "CARGO_MANIFEST_DIR" for asset root (#1950)

This was nowhere documented inside Bevy.
Should I also mention the use case of debugging a project?

Closes #810

Co-authored-by: MinerSebas <66798382+MinerSebas@users.noreply.github.com>

Author: MinerSebas

crates/bevy_asset/src/asset_server.rs

@@ -212,6 +212,18 @@ impl AssetServer {
         load_state
     }
 
+    /// Loads an Asset at the provided relative path.
+    ///
+    /// The absolute Path to the asset is "ROOT/ASSET_FOLDER_NAME/path".
+    ///
+    /// By default the ROOT is the directory of the Application, but this can be overridden by
+    /// setting the `"CARGO_MANIFEST_DIR"` environment variable (see https://doc.rust-lang.org/cargo/reference/environment-variables.html)
+    /// to another directory. When the application  is run through Cargo, then
+    /// `"CARGO_MANIFEST_DIR"` is automatically set to the root folder of your crate (workspace).
+    ///
+    /// The name of the asset folder is set inside the
+    /// [`AssetServerSettings`](crate::AssetServerSettings) resource. The default name is
+    /// `"assets"`.
     pub fn load<'a, T: Asset, P: Into<AssetPath<'a>>>(&self, path: P) -> Handle<T> {
         self.load_untyped(path).typed()
     }

examples/asset/asset_loading.rs

@@ -15,8 +15,14 @@ fn setup(
     meshes: Res<Assets<Mesh>>,
     mut materials: ResMut<Assets<StandardMaterial>>,
 ) {
-    // 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"
+    // By default AssetServer will load assets from inside the "assets" folder.
+    // For example, the next line will load "ROOT/assets/models/cube/cube.gltf#Mesh0/Primitive0",
+    // where "ROOT" is the directory of the Application.
+    //
+    // This can be overridden by setting the "CARGO_MANIFEST_DIR" environment variable (see
+    // https://doc.rust-lang.org/cargo/reference/environment-variables.html)
+    // to another directory. When the Application is run through Cargo, "CARGO_MANIFEST_DIR" is
+    // automatically set to your crate (workspace) root directory.
     let cube_handle = asset_server.load("models/cube/cube.gltf#Mesh0/Primitive0");
     let sphere_handle = asset_server.load("models/sphere/sphere.gltf#Mesh0/Primitive0");