Add entry to v1.139.0 upgrade notes about appservices and /register requests

Author: anoadragon453

docs/upgrade.md

@@ -125,6 +125,21 @@ Ubuntu 24.10 Oracular Oriole [has been end-of-life since 10 Jul
 2025](https://endoflife.date/ubuntu). This release drops support for Ubuntu
 24.10, and in its place adds support for Ubuntu 25.04 Plucky Puffin.
 
+## `/register` requests from old application service implementations may break when using MAS
+
+Application Services that do not set `inhibit_login=true` when calling `POST
+/_matrix/client/v3/register` will receive the error
+`IO.ELEMENT.MSC4190.M_APPSERVICE_LOGIN_UNSUPPORTED` in response. This is a
+result of [MSC4190: Device management for application
+services](https://github.com/matrix-org/matrix-spec-proposals/pull/4190) which
+adds new endpoints for application services to create encryption-ready devices
+with other than `/login` or `/register` without `inhibit_login=true`.
+
+If an application service you use starts to fail with the mentioned error,
+ensure it is up to date. If it is, then kindly let the author know that they
+need to update their implementation to call `/register` with
+`inhibit_login=true`.
+
 # Upgrading to v1.136.0
 
 ## Deprecate `run_as_background_process` exported as part of the module API interface in favor of `ModuleApi.run_as_background_process`