docs: Add hybrid search documentation for Elasticsearch vector store

src/oss/javascript/integrations/vectorstores/elasticsearch.mdx

@@ -108,6 +108,62 @@ const clientArgs: ElasticClientArgs = {
 const vectorStore = new ElasticVectorSearch(embeddings, clientArgs);
 ```
 
+## Hybrid search
+
+<Tip>
+Hybrid search requires Elasticsearch 8.9+ for RRF (Reciprocal Rank Fusion) support.
+</Tip>
+
+Hybrid search combines kNN vector search with BM25 full-text search using Reciprocal Rank Fusion (RRF) to improve search relevance. This is useful when you want to leverage both semantic similarity and keyword matching.
+
+To enable hybrid search, pass a `HybridRetrievalStrategy` to the constructor:
+
+```typescript
+import {
+  ElasticVectorSearch,
+  HybridRetrievalStrategy,
+  type ElasticClientArgs,
+} from "@langchain/community/vectorstores/elasticsearch";
+
+const hybridVectorStore = new ElasticVectorSearch(embeddings, {
+  client: new Client(config),
+  indexName: "test_hybrid_search",
+  strategy: new HybridRetrievalStrategy({
+    rankWindowSize: 100,  // Number of documents to consider for RRF
+    rankConstant: 60,     // RRF constant for score normalization
+    textField: "text",    // Field to use for BM25 full-text search
+  }),
+});
+```
+
+Once configured, hybrid search is automatically used for all similarity searches:
+
+```typescript
+// This now uses hybrid search (vector + BM25 + RRF)
+const results = await hybridVectorStore.similaritySearch(
+  "how to prevent muscle soreness while running",
+  5
+);
+```
+
+### Elasticsearch 9.2+ compatibility
+
+Elasticsearch 9.2+ excludes vectors from `_source` by default for performance. If you need vector data in search responses, set `includeSourceVectors`:
+
+```typescript
+const hybridVectorStore = new ElasticVectorSearch(embeddings, {
+  client: new Client(config),
+  indexName: "test_hybrid_search",
+  strategy: new HybridRetrievalStrategy({
+    includeSourceVectors: true,  // Include vectors in responses for ES 9.2+
+  }),
+});
+```
+
+<Note>
+The `includeSourceVectors` parameter is not supported on Elasticsearch versions earlier than 8.19.
+</Note>
+
 ## Manage vector store
 
 ### Add items to vector store