Add message/client-summary endpoint docs

Author: Vlad Velici

src/pages/docs/api/rest-api.mdx

@@ -991,7 +991,7 @@ Example request:
 
 <Code>
 ```shell
-curl https://rest.ably.io/channels/rest-example/messages/01726585978590-001@abcdefghij:001/annotations?limit=100 \
+curl https://rest.ably.io/channels/mutable:channel/messages/01726585978590-001@abcdefghij:001/annotations?limit=100 \
  -u "{{API_KEY}}"
 ```
 </Code>
@@ -1032,6 +1032,40 @@ A successful request returns a [paginated response](#pagination) with an array c
 ```
 </Code>
 
+### Retrieve annotation summary for a client <a id="annotations-client-summary" />
+
+#### GET rest.ably.io/channels/\{channelId\}/messages/\{messageSerial\}/client-summary
+
+Retrieve the annotation summary for a specific `clientId` for a specific message identified by its serial.
+
+Example request:
+
+<Code>
+```shell
+curl https://rest.ably.io/channels/mutable:channel/messages/01726585978590-001@abcdefghij:001/client-summary?forClientId=client1 \
+ -u "{{API_KEY}}"
+```
+</Code>
+
+##### Parameters
+
+| Parameter | Description | Type |
+|-----------|-------------|------|
+| forClientId | The `clientId` to retrieve the annotation summary for. Defaults to the `clientId` of the authenticated client. | string |
+
+##### Options
+
+| Option | Value |
+|--------|-------|
+| Accept | `application/json` (the default), `application/x-msgpack` |
+| Auth required | yes ([basic](#basic-authentication) or [token](#token-authentication)) |
+
+##### Returns
+
+A successful request returns an object containing the annotation summary for the specified message, filtered to only include the summary entries relevant to the requested `clientId`.
+
+The summary is an object whose keys are [annotation types](/docs/messages/annotations#annotation-types), and the values are aggregated summaries for that annotation type.
+
 ### Retrieve instantaneous presence status for a channel <a id="presence" />
 
 #### GET main.realtime.ably.net/channels/\{channelId\}/presence