Add comments explaining why the reloading code is so complex. (#22264)

# Objective

- Fixes #21222.

## Solution

- Give an explanation, and add a TODO. Note I didn't write this code, so
I'm "guessing" on the rationale - but it's an educated guess!

Author: andriyDev

crates/bevy_asset/src/server/mod.rs

@@ -887,6 +887,8 @@ impl AssetServer {
             .spawn(async move {
                 let mut reloaded = false;
 
+                // First, try to reload the asset for any handles to that path. This will try both
+                // root assets and subassets.
                 let requests = server
                     .read_infos()
                     .get_path_handles(&path)
@@ -902,6 +904,14 @@ impl AssetServer {
                     }
                 }
 
+                // If the above section failed, and there are still living subassets (aka we should
+                // reload), then just try doing an untyped load. This helps catch cases where the
+                // root asset has been dropped, but all its subassets are still being used (in which
+                // case the above section would have tried to find the loader with the root asset's
+                // type and loaded it). Hopefully the untyped load will find the right loader and
+                // reload all the subassets (though this is not guaranteed).
+                // TODO: Make sure we use the same loader as the original load (e.g., by storing a
+                // map from asset index to loader).
                 if !reloaded && server.read_infos().should_reload(&path) {
                     server.write_infos().stats.started_load_tasks += 1;
                     match server.load_internal(None, path.clone(), true, None).await {