Clarify that static_folder must be a relative path

Also hint at `root_path` parameter rather than mentioning in plain text.

Author: jeenuv

src/flask/app.py

@@ -170,8 +170,9 @@ class Flask(_PackageBoundObject):
                             static files on the web.  Defaults to the name
                             of the `static_folder` folder.
     :param static_folder: the folder with static files that should be served
-                          at `static_url_path`.  Defaults to the ``'static'``
-                          folder in the root path of the application.
+                          at `static_url_path`. Must be a relative path under
+                          `root_path` of the application. Defaults to
+                          ``'static'``.
     :param static_host: the host to use when adding the static route.
         Defaults to None. Required when using ``host_matching=True``
         with a ``static_folder`` configured.