mention MSC 3051 for proposed multiple relations

proposals/2674-event-relationships.md

@@ -34,8 +34,10 @@ where the subject is the relation event itself; the verb is the `rel_type`
 field of the `m.relates_to` and the object is the `event_id` field.
 
 We consciously do not support multiple different relations within a single event,
-in order to keep the API simple. A future MSC can propose a change to add support
+in order to keep the API simple. Another MSC,
-for multiple relations if it turns out that this would facilitate certain use cases.
+like [MSC 3051](https://github.com/matrix-org/matrix-doc/pull/3051),
+can propose a change to add support for multiple relations if it turns out that
+this would facilitate certain use cases.
 
 A `rel_type` of `m.reference` is defined for future handling replies and
 threading. This let you define an event which references an existing