agents: add callout around defining tools (#1424)

Author: ccurme

src/oss/langchain/agents.mdx

@@ -233,6 +233,13 @@ For more information, see [Tools](/oss/langchain/tools).
 Pass a list of tools to the agent.
 
 :::python
+
+<Tip>
+Tools can be specified as plain Python functions or coroutines.
+
+The [tool decorator](/oss/langchain/tools#create-tools) can be used to customize tool names, descriptions, argument schemas, and other properties.
+</Tip>
+
 ```python wrap
 from langchain.tools import tool
 from langchain.agents import create_agent