add some clarifications

proposals/3911-linking-media-to-events.md

@@ -148,6 +148,11 @@ to a user if the user can see the corresponding event.
    This "copy" api is to be used by clients when forwarding events with media
    attachments.
 
+   (This mechanism, rather than just allowing clients to attach media to multiple
+   events, is necessary to ensure that the list of events attached a given piece of
+   media does not grow over time, which would make it difficult for servers to
+   reliably cache media and impose the correct access restrictions.)
+
 5. Autogenerated `m.room.member` events
 
    Servers will generate `m.room.member` events with an `avatar_url` whenever
@@ -225,6 +230,10 @@ Fixes [synapse#1263](https://github.com/matrix-org/synapse/issues/1263).
 * Since each `m.room.member` references the avatar separately, changing your
   avatar will cause an even bigger traffic storm if you're in a lot of rooms.
 
+  In addition, this could cause duplication of media in the remote media cache,
+  if the implementation does not take steps to deduplicate (eg, storing media
+  by content hash rather than media id).
+
 ## Alternatives
 
 * Have the "upload" endpoint return a nonce, which can then be used in the