Update docstring in _vary_network_conn (#588)

Author: nathanjmcdougall

tests/conftest.py

@@ -98,7 +98,11 @@ def _online_status(request: pytest.FixtureRequest) -> NetworkConn:
 
 @pytest.fixture(scope="session")
 def _vary_network_conn(_online_status: NetworkConn) -> Generator[None, None, None]:
-    """Fixture to vary the network connection; returns True if offline."""
+    """Fixture to vary the network connection.
+
+    Use `usethis._config.usethis_config` to check whether things are in offline
+    model, since this fixture does not return anything.
+    """
     offline = _online_status is NetworkConn.OFFLINE
 
     usethis_config.offline = offline