Doc AssetServer::load() is async (#3129)

# Objective

Document that `AssetServer::load()` is asynchronous.

## Solution

Document that `AssetServer::load()` is asynchronous, and that the asset
will not be immediately available once the call returns. Instead,
explain that the user must call `AssetServer::get_load_state()` to
monitor the loading state of an asset.

Author: djeedai

crates/bevy_asset/src/asset_server.rs

@@ -213,7 +213,7 @@ impl AssetServer {
         load_state
     }
 
-    /// Loads an Asset at the provided relative path.
+    /// Queue an [`Asset`] at the provided relative path for asynchronous loading.
     ///
     /// The absolute Path to the asset is "ROOT/ASSET_FOLDER_NAME/path".
     ///
@@ -226,6 +226,10 @@ impl AssetServer {
     /// The name of the asset folder is set inside the
     /// [`AssetServerSettings`](crate::AssetServerSettings) resource. The default name is
     /// `"assets"`.
+    ///
+    /// The asset is loaded asynchronously, and will generally not be available by the time
+    /// this calls returns. Use [`AssetServer::get_load_state`] to determine when the asset is
+    /// effectively loaded and available in the [`Assets`] collection.
     #[must_use = "not using the returned strong handle may result in the unexpected release of the asset"]
     pub fn load<'a, T: Asset, P: Into<AssetPath<'a>>>(&self, path: P) -> Handle<T> {
         self.load_untyped(path).typed()