Clarify that login flows must be completed in order

Fixes https://github.com/matrix-org/matrix-doc/issues/1134

Evidence of this being the case is shown here: https://github.com/matrix-org/synapse/pull/5174

changelogs/client_server/newsfragments/2042.clarification

@@ -0,0 +1 @@
+Clarify that login flows are meant to be completed in order.

specification/client_server_api.rst

@@ -406,8 +406,9 @@ an additional stage. This exchange continues until the final success.
 
 For each endpoint, a server offers one or more 'flows' that the client can use
 to authenticate itself. Each flow comprises a series of stages, as described
-above. The client is free to choose which flow it follows. When all stages in a
-flow are complete, authentication is complete and the API call succeeds.
+above. The client is free to choose which flow it follows, however the flow's
+stages must be completed in order. When all stages in a flow are complete,
+authentication is complete and the API call succeeds.
 
 User-interactive API in the REST API
 <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<