HSEARCH-4950 Add new knn option to the documentation

documentation/src/main/asciidoc/public/reference/_mapping-directfieldmapping.adoc

@@ -136,8 +136,6 @@ which generally gives much more flexibility.
 +
 include::../components/_incubating-warning.adoc[]
 +
-WARNING: Vector fields are only supported by the <<backend-lucene, Lucene backend>> for now.
-+
 Specific field type for vector fields to be used in a <<search-dsl-predicate-knn,vector search>>.
 +
 Vector fields accept values of type `float[]` or `byte[]` and *require* that

documentation/src/main/asciidoc/public/reference/_search-dsl-predicate.adoc

@@ -1504,6 +1504,40 @@ include::{sourcedir}/org/hibernate/search/documentation/search/predicate/Predica
 ----
 ====
 
+[[search-dsl-predicate-knn-limitations]]
+=== Backend specifics and limitations
+
+With the Elasticsearch backend a `knn` predicate can only be added as a top-level predicate,
+i.e. a predicate directly passed to a where clause of a search query, or if a disjunction of multiple `knn` predicates is required,
+they can be supplied as `should` clauses of a top-level <<search-dsl-predicate-boolean,`boolean` predicate>>
+(the same can be achieved by using an <<search-dsl-predicate-or,`or` predicate>>).
+Any other usages of a `knn` predicate, with this backend, would lead to an exception being thrown.
+
+.Multiple `knn` predicates added via `should` clauses
+====
+[source, JAVA, indent=0, subs="+callouts"]
+----
+include::{sourcedir}/org/hibernate/search/documentation/search/predicate/PredicateDslIT.java[tags=knn-should]
+----
+====
+
+The Elasticsearch backend also allows configuring a backend-specific `knn` predicate option: the number of candidates.
+This option specifies a number of approximate nearest neighbor candidates to be found on each shard,
+then the results from each shard are merged and the top `k` are selected.
+See the Elasticsearch documentation for more details.
+To access this option an Elasticsearch extension should be used:
+
+.Setting a number of candidates Elasticsearch-specific knn option
+====
+[source, JAVA, indent=0, subs="+callouts"]
+----
+include::{sourcedir}/org/hibernate/search/documentation/search/predicate/PredicateDslIT.java[tags=knn-candidates]
+----
+<1> Get an extended, Elasticsearch-specific, predicate factory.
+<2> Build a `knn` predicate as ususal.
+<3> Provide an Elasticsearch-specific predicate option.
+====
+
 [[search-dsl-predicate-knn-other]]
 === Other options
 

documentation/src/test/java/org/hibernate/search/documentation/search/predicate/Book.java

@@ -44,6 +44,7 @@ public class Book {
 	private String comment;
 
 	private float[] coverImageEmbeddings;
+	private float[] alternativeCoverImageEmbeddings;
 
 	@ManyToMany
 	@IndexedEmbedded(structure = ObjectStructure.NESTED)

documentation/src/test/java/org/hibernate/search/documentation/search/predicate/PredicateDslIT.java

@@ -18,6 +18,7 @@
 
 import jakarta.persistence.EntityManagerFactory;
 
+import org.hibernate.search.backend.elasticsearch.ElasticsearchExtension;
 import org.hibernate.search.documentation.testsupport.BackendConfigurations;
 import org.hibernate.search.documentation.testsupport.DocumentationSetupHelper;
 import org.hibernate.search.engine.search.common.BooleanOperator;
@@ -32,6 +33,7 @@
 import org.hibernate.search.mapper.orm.mapping.HibernateOrmSearchMappingConfigurer;
 import org.hibernate.search.mapper.orm.scope.SearchScope;
 import org.hibernate.search.mapper.orm.session.SearchSession;
+import org.hibernate.search.mapper.pojo.mapping.definition.programmatic.TypeMappingStep;
 import org.hibernate.search.util.common.data.RangeBoundInclusion;
 import org.hibernate.search.util.impl.integrationtest.common.extension.BackendConfiguration;
 
@@ -64,10 +66,14 @@ void setup() {
 		if ( BackendConfiguration.isLucene() ) {
 			setupContext.withProperty(
 					HibernateOrmMapperSettings.MAPPING_CONFIGURER,
-					(HibernateOrmSearchMappingConfigurer) context -> context.programmaticMapping()
-							.type( Book.class )
-							.property( "coverImageEmbeddings" )
-							.vectorField( 128 )
+					(HibernateOrmSearchMappingConfigurer) context -> {
+						TypeMappingStep book = context.programmaticMapping()
+								.type( Book.class );
+						book.property( "coverImageEmbeddings" )
+								.vectorField( 128 );
+						book.property( "alternativeCoverImageEmbeddings" )
+								.vectorField( 128 );
+					}
 			);
 		}
 		entityManagerFactory = setupContext.setup( Book.class, Author.class, EmbeddableGeoPoint.class );
@@ -1143,6 +1149,49 @@ void knn() {
 					.extracting( Book::getId )
 					.containsExactlyInAnyOrder( BOOK1_ID, BOOK2_ID, BOOK3_ID );
 		} );
+
+		withinSearchSession( searchSession -> {
+			// tag::knn-should[]
+			float[] coverImageEmbeddingsVector = /*...*/
+					// end::knn-should[]
+					new float[128];
+			// tag::knn-should[]
+			float[] alternativeCoverImageEmbeddingsVector = /*...*/
+					// end::knn-should[]
+					new float[128];
+			// tag::knn-should[]
+			List<Book> hits = searchSession.search( Book.class )
+					.where( f -> f.bool()
+							.should( f.knn( 10 ).field( "coverImageEmbeddings" ).matching( coverImageEmbeddingsVector ) )
+							.should( f.knn( 5 ).field( "alternativeCoverImageEmbeddings" )
+									.matching( alternativeCoverImageEmbeddingsVector ) )
+					)
+					.fetchHits( 20 );
+			// end::knn-should[]
+			assertThat( hits )
+					.extracting( Book::getId )
+					.containsExactlyInAnyOrder( BOOK1_ID, BOOK2_ID, BOOK3_ID, BOOK4_ID );
+		} );
+
+
+		if ( BackendConfiguration.isElasticsearch() ) {
+			withinSearchSession( searchSession -> {
+				// tag::knn-candidates[]
+				float[] coverImageEmbeddingsVector = /*...*/
+						// end::knn-candidates[]
+						new float[128];
+				// tag::knn-candidates[]
+				List<Book> hits = searchSession.search( Book.class )
+						.where( f -> f.extension( ElasticsearchExtension.get() ) // <1>
+								.knn( 5 ).field( "coverImageEmbeddings" ).matching( coverImageEmbeddingsVector ) // <2>
+								.numberOfCandidates( 15 ) )// <3>
+						.fetchHits( 20 );
+				// end::knn-candidates[]
+				assertThat( hits )
+						.extracting( Book::getId )
+						.containsExactlyInAnyOrder( BOOK1_ID, BOOK2_ID, BOOK3_ID, BOOK4_ID );
+			} );
+		}
 	}
 
 	private MySearchParameters getSearchParameters() {