Add info for LLM Tool Selector (#1401)

Author: lnhsingh

src/oss/langchain/middleware/built-in.mdx

@@ -1069,6 +1069,8 @@ Use an LLM to intelligently select relevant tools before calling the main model.
 - Reducing token usage by filtering irrelevant tools.
 - Improving model focus and accuracy.
 
+This middleware uses structured output to ask an LLM which tools are most relevant for the current query. The structured output schema defines the available tool names and descriptions. Model providers often add this structured output information to the system prompt behind the scenes.
+
 :::python
 **API reference:** @[`LLMToolSelectorMiddleware`]