Update Javadoc for BatchMapping

Closes gh-166

spring-graphql-docs/src/docs/asciidoc/index.adoc

@@ -290,11 +290,11 @@ default it is marked as `INTERNAL_ERROR`.
 
 Given a `Book` and its `Author`, we can create one `DataFetcher` for books and another
 for the author of a book. This means books and authors aren't automatically loaded
-together, which enables queries to select the subset of data they need. However, when
-loading multiple books, the author for each book is loaded individually, and this is
-an issue known as the N+1 select problem.
+together, which enables queries to select the subset of data they need. However, loading
+multiple books, results in loading each author individually, and this is a performance
+issue known as the N+1 select problem.
 
-To address the issue, GraphQL Java provides a
+GraphQL Java provides a
 https://www.graphql-java.com/documentation/v16/batching/[batching feature] that allows
 related entities, such as the authors for all books, to be loaded together. This is how
 the underlying mechanism works in GraphQL Java:
@@ -310,7 +310,7 @@ defers until it is ready to batch load all related entities as one.
  - `DataLoader` additionally maintains a cache of previously loaded entities that can
 further improve efficiency when the same entity is in multiple places of the response.
 
-Spring GraphQL exposes a `BatchLoaderRegistry` that accepts and stores registrations of
+Spring GraphQL provides a `BatchLoaderRegistry` that accepts and stores registrations of
 batch loading functions. The `ExecutionGraphQlService` accepts the registry as input and
 uses it to make per request `DataLoader` registrations. A `DataFetcher` then looks up the
 `DataLoader` for an entity and uses it to load instances, or in an annotated controller,

spring-graphql/src/main/java/org/springframework/graphql/data/method/annotation/BatchMapping.java

@@ -29,7 +29,7 @@ import org.springframework.core.annotation.AliasFor;
  *
  * <pre class="code">
  * &#064;BatchMapping
- * public Flux&lt;Author&gt; author(List&lt;Book&gt; books) {
+ * public Mono&lt;Map&lt;Book, Author&gt;&gt; author(List&lt;Book&gt; books) {
  *     // ...
  * }
  * </pre>
@@ -47,17 +47,23 @@ import org.springframework.core.annotation.AliasFor;
  * public class BookController {
  *
  *     public BookController(BatchLoaderRegistry registry) {
- *         registry.forTypePair(Long.class, Author.class).registerBatchLoader((ids, environment) -> ...);
+ *         registry.forTypePair(Long.class, Author.class).registerMappedBatchLoader((ids, environment) -> ...);
  *     }
  *
  *     &#064;SchemaMapping
- *     public Author author(Book book, DataLoader&lt;Long, Author&gt; dataLoader) {
+ *     public CompletableFuture&lt;Author&gt; author(Book book, DataLoader&lt;Long, Author&gt; dataLoader) {
  *         return dataLoader.load(book.getAuthorId());
  *     }
  *
  * }
  * </pre>
  *
+ * <p>In addition to returning {@code Mono<Map<K,T>>}, an {@code @BatchMapping}
+ * method can also return {@code Flux<T>}. However, in that case the returned
+ * sequence of values must match the number and order of the input keys. See
+ * {@link org.dataloader.BatchLoader} and {@link org.dataloader.MappedBatchLoader}
+ * for more details.
+ *
  * @author Rossen Stoyanchev
  * @since 1.0.0
  */