Allow admins to see soft failed events (if they want to) (#18238)

docs/SUMMARY.md

@@ -74,6 +74,7 @@
       - [Users](admin_api/user_admin_api.md)
       - [Server Version](admin_api/version_api.md)
       - [Federation](usage/administration/admin_api/federation.md)
+      - [Client-Server API Extensions](admin_api/client_server_api_extensions.md)
     - [Manhole](manhole.md)
     - [Monitoring](metrics-howto.md)
       - [Reporting Homeserver Usage Statistics](usage/administration/monitoring/reporting_homeserver_usage_statistics.md)

docs/admin_api/client_server_api_extensions.md

@@ -0,0 +1,25 @@
+# Client-Server API Extensions
+
+Server administrators can set special account data to change how the Client-Server API behaves for
+their clients. Setting the account data, or having it already set, as a non-admin has no effect.
+
+All configuration options can be set through the `io.element.synapse.admin_client_config` global 
+account data on the admin's user account. 
+
+Example:
+```
+PUT /_matrix/client/v3/user/{adminUserId}/account_data/io.element.synapse.admin_client_config
+{
+    "return_soft_failed_events": true
+}
+```
+
+## See soft failed events
+
+Learn more about soft failure from [the spec](https://spec.matrix.org/v1.14/server-server-api/#soft-failure).
+
+To receive soft failed events in APIs like `/sync` and `/messages`, set `return_soft_failed_events`
+to `true` in the admin client config. When `false`, the normal behaviour of these endpoints is to
+exclude soft failed events.
+
+Default: `false`