Update sample env file and documentation

- Change COSINE_THRESHOLD to 0.4
- Adjust TOP_K to 50
- Enhance API README details

.env.example

@@ -14,8 +14,8 @@ MAX_EMBED_TOKENS=8192
 #HISTORY_TURNS=3
 #CHUNK_SIZE=1200
 #CHUNK_OVERLAP_SIZE=100
-#COSINE_THRESHOLD=0.2
-#TOP_K=50
+#COSINE_THRESHOLD=0.4   # 0.2 while not running API server
+#TOP_K=50               # 60  while not running API server
 
 # LLM Configuration (Use valid host. For local services, you can use host.docker.internal)
 # Ollama example