Clarify what to do if no (new) docs

proposals/2140-terms-of-service-2.md

@@ -174,11 +174,13 @@ documents that the user must agree to in order to use the service.
 
 It then cross-references this set of documents against the `m.accepted_terms`
 account data and presents to the user any documents that they have not already
-agreed to, along with UI for them to indicate their agreement. Once the user
-has indicated their agreement, it adds these URLs to `m.accepted_terms` account
-data. Once this has succeeded, then, and only then, must the client perform
-OpenID authentication, getting a token from the Homeserver and submitting this
-to the service using the `register` endpoint.
+agreed to, along with UI for them to indicate their agreement. If there are no
+such documents (ie. if the `policies` dict is empty or the user has already
+agreed to all documents) the client proceeds to perform the OpenID
+registration. Once the user has indicated their agreement, it adds these URLs
+to `m.accepted_terms` account data. Once this has succeeded, then, and only
+then, must the client perform OpenID authentication, getting a token from the
+Homeserver and submitting this to the service using the `register` endpoint.
 
 Having done this, if the user agreed to any new documents, it performs a `POST
 $prefix/terms` request to signal to the server the set of documents that the