Add note for using xfail on test for mqtt_statestream (#91999)

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>

tests/components/mqtt_statestream/test_init.py

@@ -96,6 +96,9 @@ async def test_setup_and_stop_waits_for_ha(
     mqtt_mock.async_publish.assert_not_called()
 
 
+# We use xfail with this test because there is an unhandled exception
+# in a background task in this test.
+# The exception is raised by mqtt.async_publish.
 @pytest.mark.xfail()
 async def test_startup_no_mqtt(
     hass: HomeAssistant, caplog: pytest.LogCaptureFixture