adding docstring for api_router param

nicegui/page.py

@@ -38,6 +38,7 @@ class page:
         :param favicon: optional relative filepath or absolute URL to a favicon (default: `None`, NiceGUI icon will be used)
         :param dark: whether to use Quasar's dark mode (defaults to `dark` argument of `run` command)
         :param response_timeout: maximum time for the decorated function to build the page (default: 3.0)
+        :param api_router: APIRouter instance to use, can be left `None` to use the default
         :param kwargs: additional keyword arguments passed to FastAPI's @app.get method
         """
         self.path = path