pubsub/messages: advice high throughput appends

src/pages/docs/messages/updates-deletes.mdx

@@ -405,6 +405,10 @@ To append data to an existing message, use the `appendMessage()` method on a RES
 
 The message is identified by its `serial`, which is populated by Ably. To append to a message, you need its serial - you can get this either from the return value of `publish()`, from a received message via subscription, or by querying history.
 
+<Aside data-type="note">
+If you are publishing appends at a high rate, use a [Realtime](/docs/api/realtime-sdk) client rather than [REST](/docs/api/rest-sdk). A Realtime client maintains a persistent connection to the Ably service, allowing you to publish at high message rates with low latencies while preserving [message delivery order](/docs/platform/architecture/message-ordering). For more information, see [Realtime and REST](/docs/basics#realtime-and-rest).
+</Aside>
+
 When Ably receives an append, it concatenates the provided data with the current latest version to calculate a full (non-incremental) version of the message. That version (with an action of `message.update`) is then used in contexts like [History](/docs/storage-history/history) and [rewind](/docs/channels/options/rewind), so that when using those APIs, you will always receive complete messages without needing to do any concatenation yourself, and can specify e.g. `rewind=10` to get the most recent 10 full, distinct messages.
 
 ### Comparison with update