Clarify that clients should replace events with the most recent replacement by origin_server_ts (#2190)

Signed-off-by: Andy Balaam <andy.balaam@matrix.org>

changelogs/internal/newsfragments/2190.clarification

@@ -0,0 +1 @@
+Clarify that clients should replace events with the most recent replacement by origin_server_ts.

content/client-server-api/modules/event_replacements.md

@@ -195,7 +195,7 @@ given event (for example, if an event is edited multiple times). These should
 be [aggregated](#aggregations-of-child-events) by the homeserver.
 
 The aggregation format of `m.replace` relationships gives the **most recent**
-replacement event, formatted [as normal](#room-event-format).
+valid replacement event, formatted [as normal](#room-event-format).
 
 The most recent event is determined by comparing `origin_server_ts`; if two or
 more replacement events have identical `origin_server_ts`, the event with the
@@ -268,6 +268,11 @@ Client authors are reminded to take note of the requirements for [Validity of
 replacement events](#validity-of-replacement-events), and to ignore any
 invalid replacement events that are received.
 
+Clients should render the content of the **most recent** valid replacement event. The
+most recent event is determined by comparing `origin_server_ts`; if two or more
+replacement events have identical `origin_server_ts`, the event with the
+lexicographically largest `event_id` is treated as more recent.
+
 ##### Permalinks
 
 When creating [links](/appendices/#uris) to events (also known as permalinks),