Add a note in `behinf-a-proxy.md`

docs/en/docs/advanced/behind-a-proxy.md

@@ -394,6 +394,14 @@ This is a more advanced use case. Feel free to skip it.
 
 By default, **FastAPI** will create a `server` in the OpenAPI schema with the URL for the `root_path`.
 
+/// note | Technical Details
+
+The `servers` property in the OpenAPI specification is optional.
+
+If you don't specify the `servers` parameter and `root_path` is equal to `/`, the `servers` property in the generated OpenAPI schema will be omitted entirely. Clients should interpret this as a single server with a `url` value of `/`.
+
+///
+
 But you can also provide other alternative `servers`, for example if you want *the same* docs UI to interact with both a staging and a production environment.
 
 If you pass a custom list of `servers` and there's a `root_path` (because your API lives behind a proxy), **FastAPI** will insert a "server" with this `root_path` at the beginning of the list.