Update docs

Author: erikjohnston

docs/admin_api/experimental_features.md

@@ -4,6 +4,7 @@ This API allows a server administrator to enable or disable some experimental fe
 basis. The currently supported features are:
 - [MSC3881](https://github.com/matrix-org/matrix-spec-proposals/pull/3881): enable remotely toggling push notifications
 for another client
+- [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575): enable experimental sliding sync support
 
 To use it, you will need to authenticate by providing an `access_token`
 for a server admin: see [Admin API](../usage/administration/admin_api/).