Add final missing migration guides (#1856)

Co-authored-by: Rich Churcher <[email protected]>

release-content/0.15/migration-guides/14082_AssetReader_read_now_returns_an_opaque_type.md

@@ -0,0 +1,38 @@
+The trait method `bevy_asset::io::AssetReader::read` (and `read_meta`) now return an opaque type instead of a boxed trait object. Implementors of these methods should change the type signatures appropriately:
+
+```rust
+impl AssetReader for MyReader {
+    // Before
+    async fn read<'a>(&'a self, path: &'a Path) -> Result<Box<Reader<'a>>, AssetReaderError> {
+        let reader = // construct a reader
+        Box::new(reader) as Box<Reader<'a>>
+    }
+
+    // After
+    async fn read<'a>(&'a self, path: &'a Path) -> Result<impl Reader + 'a, AssetReaderError> {
+        // create a reader
+    }
+}
+```
+
+`bevy::asset::io::Reader` is now a trait, rather than a type alias for a trait object. Implementors of `AssetLoader::load` will need to adjust the method signature accordingly:
+
+```rust
+impl AssetLoader for MyLoader {
+    async fn load<'a>(
+        &'a self,
+        // Before:
+        reader: &'a mut bevy::asset::io::Reader,
+        // After:
+        reader: &'a mut dyn bevy::asset::io::Reader,
+        _: &'a Self::Settings,
+        load_context: &'a mut LoadContext<'_>,
+    ) -> Result<Self::Asset, Self::Error> {
+}
+```
+
+Additionally, implementors of `AssetReader` that return a type implementing `futures_io::AsyncRead` and `AsyncSeek` might need to explicitly implement `bevy::asset::io::Reader` for that type.
+
+```rust
+impl bevy::asset::io::Reader for MyAsyncReadAndSeek {}
+```

release-content/0.15/migration-guides/15090_Deprecate_LoadAndSave_Asset_Processor.md

@@ -0,0 +1,2 @@
+- Replace `LoadAndSave<L, S>` with `LoadTransformAndSave<L, IdentityAssetTransformer<<L as AssetLoader>::Asset>, S>`
+- Replace `LoadAndSaveSettings<L, S>` with `LoadTransformAndSaveSettings<L, (), S>`

release-content/0.15/migration-guides/_guides.toml

@@ -1315,5 +1315,17 @@ file_name = "13760_Add_Display_implementation_to_DebugName.md"
 [[guides]]
 title = "Remove `accesskit` re-export from `bevy_a11y`"
 prs = [16257]
-areas = []
+areas = ["Accessibility"]
 file_name = "16257_Remove_accesskit_reexport_from_bevy_a11y.md"
+
+[[guides]]
+title = "Deprecate `LoadAndSave` Asset Processor"
+prs = [15090]
+areas = ["Asset"]
+file_name = "15090_Deprecate_LoadAndSave_Asset_Processor.md"
+
+[[guides]]
+title = "`AssetReader::read` now returns an opaque type"
+prs = [14082]
+areas = ["Asset"]
+file_name = "14082_AssetReader_read_now_returns_an_opaque_type.md"