doc: sample prompt instruction for rag as tool

docs/rag_guide.md

@@ -18,6 +18,7 @@ This document explains how to configure and customize your RAG pipeline using th
 * [Configure Vector Store and Embedding Model](#configure-vector-store-and-embedding-model)
 * [Add an Inference Model (LLM)](#add-an-inference-model-llm)
 * [Complete Configuration Reference](#complete-configuration-reference)
+* [System Prompt Guidance for RAG (as a tool)](#system-prompt-guidance-for-rag-as-a-tool)
 * [References](#references)
 
 
@@ -350,6 +351,18 @@ vector_dbs:
   vector_db_id: openshift-index  # This ID was defined during index generation
 ```
 
+---
+
+# System Prompt Guidance for RAG (as a tool)
+
+When using RAG, the `knowledge_search` tool must be explicitly referenced in your system prompt. Without clear instructions, models may inconsistently use the tool.
+
+**Tool-Aware sample instruction**:
+```text
+You are a helpful assistant with access to a 'knowledge_search' tool. When users ask questions, ALWAYS use the knowledge_search tool first to find accurate information from the documentation before answering.
+```
+---
+
 # References
 
 * [Llama Stack - RAG](https://llama-stack.readthedocs.io/en/latest/building_applications/rag.html)