Add a migration guide.

Author: andriyDev

release-content/migration-guides/reflect_asset_asset_ids.md

@@ -0,0 +1,25 @@
+---
+title: `ReflectAsset` now uses `UntypedAssetId` instead of `UntypedHandle`.
+pull_requests: [19606]
+---
+
+Previously, `ReflectAsset` methods all required having `UntypedHandle`. The only way to get an
+`UntypedHandle` through this API was with `ReflectAsset::add`. `ReflectAsset::ids` was not very
+useful in this regard.
+
+Now, all methods have been changed to accept `impl Into<UntypedAssetId>`, which matches our regular
+`Assets<T>` API. This means you may need to change how you are calling these methods.
+
+For example, if your code previously looked like:
+
+```rust
+let my_handle: UntypedHandle;
+let my_asset = reflect_asset.get_mut(world, my_handle).unwrap();
+```
+
+You can migrate it to:
+
+```rust
+let my_handle: UntypedHandle;
+let my_asset = reflect_asset.get_mut(world, &my_handle).unwrap();
+```