docs: add note about @types/react for default JSX mode (#3040)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

runtime/reference/jsx.md

@@ -29,6 +29,17 @@ options by default:
 }
 ```
 
+:::note
+
+When using the default `"react"` mode with React, you'll need to add
+`@types/react` for type checking to work (e.g.,
+`"@types/react": "npm:@types/react"` in your `deno.json` imports). Without it
+you'll get errors like `JSX element implicitly has type 'any'`. Consider using
+the [automatic runtime](#jsx-automatic-runtime-recommended) instead, which is
+the recommended approach.
+
+:::
+
 Using the `"react"` option will convert JSX into the following JavaScript code:
 
 ```jsx