Export WebSocketDisconnect and add example handling disconnections to docs (#1822)

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

tests/test_tutorial/test_websockets/test_tutorial003.py

@@ -0,0 +1,22 @@
+from fastapi.testclient import TestClient
+
+from docs_src.websockets.tutorial003 import app
+
+client = TestClient(app)
+
+
+def test_websocket_handle_disconnection():
+    with client.websocket_connect("/ws/1234") as connection, client.websocket_connect(
+        "/ws/5678"
+    ) as connection_two:
+        connection.send_text("Hello from 1234")
+        data1 = connection.receive_text()
+        assert data1 == "You wrote: Hello from 1234"
+        data2 = connection_two.receive_text()
+        client1_says = "Client #1234 says: Hello from 1234"
+        assert data2 == client1_says
+        data1 = connection.receive_text()
+        assert data1 == client1_says
+        connection_two.close()
+        data1 = connection.receive_text()
+        assert data1 == "Client #5678 left the chat"