Fix docstring of `servers` parameter

fastapi/applications.py

@@ -301,7 +301,11 @@ class FastAPI(Starlette):
                 browser tabs open). Or if you want to leave fixed the possible URLs.
 
                 If the servers `list` is not provided, or is an empty `list`, the
-                default value would be a `dict` with a `url` value of `/`.
+                `servers` property in the generated OpenAPI will be:
+
+                * a `dict` with a `url` value of application's mounting point
+                (`root_path`) if it's different from `/`.
+                * otherwise, `servers` property will be omitted from the OpenAPI schema.
 
                 Each item in the `list` is a `dict` containing: