Specify that appservice login and register fail on incorrect as_tokens

Signed-off-by: Tulir Asokan <tulir@maunium.net>

changelogs/application_service/newsfragments/1744.clarification

@@ -0,0 +1 @@
+Clarify that the `/login` and `/register` endpoints should fail when using the `m.login.application_service` login type without a valid `as_token`.

content/application-service-api.md

@@ -436,6 +436,10 @@ an application service-defined namespace will receive the same
 `M_EXCLUSIVE` error code, but only if the application service has
 defined the namespace as `exclusive`.
 
+If either endpoint is called with the `m.login.application_service` login type,
+but without a valid `as_token`, the endpoints will return `M_MISSING_TOKEN` or
+`M_UNKNOWN_TOKEN`.
+
 #### Pinging
 
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