Add information from MSC2604: Parameters for Login Fallback to the spec.

changelogs/client_server/newsfragments/2709.new

@@ -0,0 +1 @@
+Document `MSC2604: Parameters for Login Fallback <https://github.com/matrix-org/matrix-doc/pull/2604>`_.

specification/client_server_api.rst

@@ -1096,6 +1096,12 @@ This returns an HTML and JavaScript page which can perform the entire login
 process. The page will attempt to call the JavaScript function
 ``window.onLogin`` when login has been successfully completed.
 
+Non-credential related parameters of the login endpoint can be provided as query
+parameters, for example the ``device_id`` or ``initial_device_display_name``.
+These are to be forwarded to the login endpoint during the login process.
+
+    GET /_matrix/static/client/login/?device_id=GHTYAJCE
+
 .. _Registration:
 
 Account registration and management