Add docs for customizng agent-as-tool (openai#504)

Co-authored-by: pakrym-oai <[email protected]>

Author: rm-openai

docs/tools.md

@@ -259,6 +259,27 @@ async def main():
     print(result.final_output)
 ```
 
+### Customizing tool-agents
+
+The `agent.as_tool` function is a convenience method to make it easy to turn an agent into a tool. It doesn't support all configuration though; for example, you can't set `max_turns`. For advanced use cases, use `Runner.run` directly in your tool implementation:
+
+```python
+@function_tool
+async def run_my_agent() -> str:
+  """A tool that runs the agent with custom configs".
+
+    agent = Agent(name="My agent", instructions="...")
+
+    result = await Runner.run(
+        agent,
+        input="...",
+        max_turns=5,
+        run_config=...
+    )
+
+    return str(result.final_output)
+```
+
 ## Handling errors in function tools
 
 When you create a function tool via `@function_tool`, you can pass a `failure_error_function`. This is a function that provides an error response to the LLM in case the tool call crashes.