Fix: OpenSearch retrieval no return & Add documentation of /retrieval (#11083)

### What problem does this PR solve?

Fix: OpenSearch retrieval no return #11006
Add documentation #11072
### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Documentation Update

---------

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>
Co-authored-by: writinwaters <93570324+writinwaters@users.noreply.github.com>

docs/references/http_api_reference.md

@@ -1840,7 +1840,7 @@ Retrieves chunks from specified datasets.
   - `"highlight"`: `boolean`
   - `"cross_languages"`: `list[string]`
   - `"metadata_condition"`: `object`
-
+  - `"use_kg"`: `boolean`
 ##### Request example
 
 ```bash
@@ -1888,6 +1888,8 @@ curl --request POST \
   The weight of vector cosine similarity. Defaults to `0.3`. If x represents the weight of vector cosine similarity, then (1 - x) is the term similarity weight.
 - `"top_k"`: (*Body parameter*), `integer`  
   The number of chunks engaged in vector cosine computation. Defaults to `1024`.
+- `"use_kg"`: (*Body parameter*), `boolean`  
+  The search includes text chunks related to the knowledge graph of the selected dataset to handle complex multi-hop queries. Defaults to `False`.
 - `"rerank_id"`: (*Body parameter*), `integer`  
   The ID of the rerank model.
 - `"keyword"`: (*Body parameter*), `boolean`  

rag/nlp/search.py

@@ -384,7 +384,7 @@ class Dealer:
                                                    rank_feature=rank_feature)
         else:
             lower_case_doc_engine = os.getenv('DOC_ENGINE', 'elasticsearch')
-            if lower_case_doc_engine == "elasticsearch":
+            if lower_case_doc_engine in ["elasticsearch","opensearch"]:
                 # ElasticSearch doesn't normalize each way score before fusion.
                 sim, tsim, vsim = self.rerank(
                     sres, question, 1 - vector_similarity_weight, vector_similarity_weight,