Document multi index query support for simplified retrievers (#134980)

Author: ioanatia

docs/reference/elasticsearch/rest-apis/retrievers.md

@@ -233,10 +233,10 @@ Note, however, that wildcard field patterns will only resolve to fields that eit
 
 ### Limitations
 
-- **Single index**: Multi-field queries currently work with single index searches only
+- **Single index**: Until 9.2, multi-field queries only work with single index searches.
 - **CCS (Cross Cluster Search)**: Multi-field queries do not support remote cluster searches
 
 ### Examples
 
 - [RRF with the multi-field query format](retrievers/retrievers-examples.md#retrievers-examples-rrf-multi-field-query-format)
-- [Linear retriever with the multi-field query format](retrievers/retrievers-examples.md#retrievers-examples-linear-multi-field-query-format)
+- [Linear retriever with the multi-field query format](retrievers/retrievers-examples.md#retrievers-examples-linear-multi-field-query-format)

x-pack/plugin/rank-rrf/src/test/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilderTests.java

@@ -445,13 +445,13 @@ public void testMultiIndexMultiFieldsParamsRewrite() {
             null
         );
 
-        // Non-default rank window size
+        // Non-default rank window size and non-default rank_constant
         retriever = new RRFRetrieverBuilder(
             null,
             List.of("field_1", "field_2", "semantic_field_1", "semantic_field_2"),
             "foo2",
             DEFAULT_RANK_WINDOW_SIZE * 2,
-            RRFRetrieverBuilder.DEFAULT_RANK_CONSTANT,
+            RRFRetrieverBuilder.DEFAULT_RANK_CONSTANT * 2,
             new float[0]
         );
         assertMultiIndexMultiFieldsParamsRewrite(