Add a warning when watch_for_changes has no effect (#3684)

# Objective

- Users can get confused when they ask for watching to be unsupported, then find it isn't supported
- Fixes https://github.com/bevyengine/bevy/issues/3683

## Solution

- Add a warning if the `watch_for_changes` call would do nothing

crates/bevy_asset/src/io/android_asset_io.rs

@@ -42,6 +42,7 @@ impl AssetIo for AndroidAssetIo {
     }
 
     fn watch_for_changes(&self) -> Result<(), AssetIoError> {
+        bevy_log::warn!("Watching for changes is not supported on Android");
         Ok(())
     }
 

crates/bevy_asset/src/io/file_asset_io.rs

@@ -104,6 +104,8 @@ impl AssetIo for FileAssetIo {
         {
             *self.filesystem_watcher.write() = Some(FilesystemWatcher::default());
         }
+        #[cfg(not(feature = "filesystem_watcher"))]
+        bevy_log::warn!("Watching for changes is not supported when the `filesystem_watcher` feature is disabled");
 
         Ok(())
     }

crates/bevy_asset/src/io/wasm_asset_io.rs

@@ -46,6 +46,7 @@ impl AssetIo for WasmAssetIo {
     }
 
     fn watch_for_changes(&self) -> Result<(), AssetIoError> {
+        bevy_log::warn!("Watching for changes is not supported in WASM");
         Ok(())
     }