login&register flow discovery by empty POST

supporting-docs/howtos/client-server-migrating-from-v1.rst

@@ -15,6 +15,10 @@ new API works. In brief, the changes are that the new version returns extra
 information in the form of the ``params`` object, and that a sequence of
 multiple calls may be statefully chained together by the ``session`` parameter.
 
+Additionally, whereas in ``v1`` the client can perform a ``GET`` request to
+discover the list of supported flows, in ``r0`` this is done by sending a
+``POST`` request with an empty data body.
+
 Deprecated Endpoints
 ====================