Enhance README with semantic ranker and agentic retrieval

Added definitions for semantic ranker and agentic retrieval.

Author: brown9804

0_Azure/3_AzureAI/0_AISearch/demos/2_Index_Increase/README.md

@@ -19,6 +19,10 @@ Last updated: 2025-07-17
 
 <img width="1562" height="628" alt="image" src="https://github.com/user-attachments/assets/2573610e-f63c-47a6-9621-2856ee218447" />
 
+> [!NOTE]
+> - Semantic ranker = a component that improves relevance. `Semantic ranker is a tool for better search.`
+> - Agentic retrieval = a pipeline that integrates the semantic ranker plus LLM reasoning to handle complex, multi-part queries and produce grounded answers. `Agentic retrieval is a system that uses that tool, but also orchestrates LLMs to make search conversational and intelligent.`
+
 <details>
 <summary><b> List of References </b> (Click to expand)</summary>