Support DataLoader in EntityMapping methods

Closes gh-1095

spring-graphql-docs/modules/ROOT/pages/federation.adoc

@@ -80,11 +80,8 @@ xref:federation.adoc#federation.entity-mapping.signature[Method Signature] for s
 method argument and return value types.
 <2> `@SchemaMapping` methods can be used for the rest of the graph.
 
-An `@EntityMapping` method can batch load federated entities of a given type. To do that,
-declare the `@Argument` method parameter as a list, and return the corresponding entity
-instances as a list in the same order.
-
-For example:
+You can load federated entities of the same type together by accepting a `List` of id's,
+and returning a `List` or `Flux` of entities:
 
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
@@ -108,6 +105,35 @@ look up the correct value in the "representation" input map. You can also set th
 argument name through the annotation.
 <2> `@BatchMapping` methods can be used for the rest of the graph.
 
+You can load federated entities with a `DataLoader`:
+
+[source,java,indent=0,subs="verbatim,quotes"]
+----
+	@Controller
+	private static class BookController {
+
+		@Autowired
+		public DataLoaderBookController(BatchLoaderRegistry registry) { // <1>
+			registry.forTypePair(Integer.class, Book.class).registerBatchLoader((bookIds, environment) -> {
+				// load entities...
+			});
+		}
+
+		@EntityMapping
+		public Future<Book> book(@Argument int id, DataLoader<Integer, Book> dataLoader) { // <2>
+			return dataLoader.load(id);
+		}
+
+		@BatchMapping
+		public Map<Book, Author> author(List<Book> books) { // <3>
+			// ...
+		}
+}
+----
+
+<1> Register a batch loader for the federated entity type.
+<2> Declare a `DataLoader` argument to the `@EntityMapping` method.
+<3> `@BatchMapping` methods can be used for the rest of the graph.
 
 
 [[federation.entity-mapping.signature]]
@@ -153,6 +179,9 @@ Entity mapping methods support the following arguments:
 | `DataFetchingEnvironment`
 | For direct access to the underlying `DataFetchingEnvironment`.
 
+| `DataLoader<I, E>`
+| To load federated entities with a `DataLoader` where `I` is the id type, and `E` is the entity type.
+
 |===
 
 `@EntityMapping` methods can return `Mono`, `CompletableFuture`, `Callable`, or the actual entity.