📝 Add external link to starlette docs for WebSocket testing (#1717 )

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>
Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

docs/en/docs/advanced/testing-websockets.md

@@ -7,3 +7,6 @@ For this, you use the `TestClient` in a `with` statement, connecting to the WebS
 ```Python hl_lines="27 28 29 30 31"
 {!../../../docs_src/app_testing/tutorial002.py!}
 ```
+
+!!! note
+    For more details, check Starlette's documentation for <a href="https://www.starlette.io/testclient/#testing-websocket-sessions" class="external-link" target="_blank">testing WebSockets</a>.