Docs: Add weight parameter documentation for Weighted RRF retriever

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

@@ -113,7 +113,9 @@ First, let’s examine how to combine two different types of queries: a `kNN` qu
 While these queries may produce scores in different ranges, we can use Reciprocal Rank Fusion (`rrf`) to combine the results and generate a merged final result list.
 
 To implement this in the retriever framework, we start with the top-level element: our `rrf` retriever.
-This retriever operates on top of two other retrievers: a `knn` retriever and a `standard` retriever. Our query structure would look like this:
+This retriever operates on top of two other retrievers: a `knn` retriever and a `standard` retriever.
+We can also specify weights to adjust the influence of each retriever on the final ranking.
+In this example, we're giving the semantic search (kNN) twice the influence of the lexical search:
 
 ```console
 GET /retrievers_example/_search
@@ -122,26 +124,32 @@ GET /retrievers_example/_search
         "rrf": {
             "retrievers": [
                 {
-                    "standard": {
-                        "query": {
-                            "query_string": {
-                                "query": "(information retrieval) OR (artificial intelligence)",
-                                "default_field": "text"
+                    "retriever": {
+                        "standard": {
+                            "query": {
+                                "query_string": {
+                                    "query": "(information retrieval) OR (artificial intelligence)",
+                                    "default_field": "text"
+                                }
                             }
                         }
-                    }
+                    },
+                    "weight": 1.0
                 },
                 {
-                    "knn": {
-                        "field": "vector",
-                        "query_vector": [
-                            0.23,
-                            0.67,
-                            0.89
-                        ],
-                        "k": 3,
-                        "num_candidates": 5
-                    }
+                    "retriever": {
+                        "knn": {
+                            "field": "vector",
+                            "query_vector": [
+                                0.23,
+                                0.67,
+                                0.89
+                            ],
+                            "k": 3,
+                            "num_candidates": 5
+                        }
+                    },
+                    "weight": 2.0
                 }
             ],
             "rank_window_size": 10,

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

@@ -6,7 +6,7 @@ applies_to:
 
 # RRF retriever [rrf-retriever]
 
-An [RRF](/reference/elasticsearch/rest-apis/reciprocal-rank-fusion.md) retriever returns top documents based on the RRF formula, equally weighting two or more child retrievers.
+An [RRF](/reference/elasticsearch/rest-apis/reciprocal-rank-fusion.md) retriever returns top documents based on the RRF formula, combining two or more child retrievers.
 Reciprocal rank fusion (RRF) is a method for combining multiple result sets with different relevance indicators into a single result set.
 
 
@@ -32,7 +32,8 @@ Combining `query` and `retrievers` is not supported.
 :   (Optional, array of retriever objects)
 
     A list of child retrievers to specify which sets of returned top documents will have the RRF formula applied to them.
-    Each child retriever carries an equal weight as part of the RRF formula. Two or more child retrievers are required.
+    Two or more child retrievers are required.
+    Each retriever can optionally include a weight to adjust its influence on the final ranking.
 
 `rank_constant`
 :   (Optional, integer)
@@ -53,6 +54,18 @@ Combining `query` and `retrievers` is not supported.
 
     Applies the specified [boolean query filter](/reference/query-languages/query-dsl/query-dsl-bool-query.md) to all of the specified sub-retrievers, according to each retriever’s specifications.
 
+Each entry in the `retrievers` array can specify the following parameters:
+
+`retriever`
+:   (Optional, a retriever object)
+
+    Specifies a child retriever. Can be omitted when using the direct format (e.g., `{ "standard": {...} }`).
+
+`weight`
+:   (Optional, float)
+
+    The weight that each score of this retriever's top docs will be multiplied with in the RRF formula. Higher values increase this retriever's influence on the final ranking. Must be non-negative. Defaults to `1.0`.
+
 ## Example: Hybrid search [rrf-retriever-example-hybrid]
 
 A simple hybrid search example (lexical search + dense vector search) combining a `standard` retriever with a `knn` retriever using RRF:
@@ -177,6 +190,71 @@ GET /restaurants/_search
 5. The rank constant for the RRF retriever.
 6. The rank window size for the RRF retriever.
 
+## Example: Weighted hybrid search [rrf-retriever-example-weighted]
+
+This example demonstrates how to use weights to adjust the influence of different retrievers in the RRF ranking.
+In this case, we're giving more importance to lexical matches (weight 2.0) compared to vector similarity (weight 1.0):
+
+```console
+GET /restaurants/_search
+{
+  "retriever": {
+    "rrf": {
+      "retrievers": [
+        {
+          "retriever": { <1>
+            "standard": {
+              "query": {
+                "multi_match": {
+                  "query": "Austria",
+                  "fields": ["city", "region"]
+                }
+              }
+            }
+          },
+          "weight": 2.0 <2>
+        },
+        {
+          "retriever": { <3>
+            "knn": {
+              "field": "vector",
+              "query_vector": [10, 22, 77],
+              "k": 10,
+              "num_candidates": 10
+            }
+          },
+          "weight": 1.0 <4>
+        }
+      ],
+      "rank_constant": 60,
+      "rank_window_size": 50
+    }
+  }
+}
+```
+% TEST[continued]
+
+1. The first retriever in weighted format.
+2. This retriever has a weight of 2.0, giving lexical matches twice the influence.
+3. The second retriever in weighted format.
+4. This retriever has a weight of 1.0 (default weight).
+
+::::{note}
+You can also mix weighted and non-weighted formats in the same query.
+Retrievers without an explicit weight default to 1.0:
+
+```json
+{
+  "rrf": {
+    "retrievers": [
+      { "standard": { "query": {...} } },  // weight defaults to 1.0
+      { "retriever": { "knn": {...} }, "weight": 2.0 }
+    ]
+  }
+}
+```
+::::
+
 ## Example: Hybrid search with sparse vectors [rrf-retriever-example-hybrid-sparse]
 
 A more complex hybrid search example (lexical search + ELSER sparse vector search + dense vector search) using RRF: