docs: update tool-reference.md

Author: ArcherGu

docs/tool-reference.md

@@ -284,7 +284,7 @@ so returned values have to JSON-serializable.
 **Parameters:**
 
 - **args** (array) _(optional)_: An optional list of arguments to pass to the function.
-- **function** (string) **(required)**: A JavaScript function to run in the currently selected page.
+- **function** (string) **(required)**: A JavaScript function to run in the currently selected page. Just define the function and let the tool do the execution.
   Example without arguments: `() => {
   return document.title
 }` or `async () => {