Fix build

- Add required documentation to `devices` field
- Import `UserId` on `sync_events`

src/r0/device/get_devices.rs

@@ -16,6 +16,7 @@ ruma_api! {
     request {}
 
     response {
+        /// A list of all registered devices for this user
         pub devices: Vec<Device>,
     }
 

src/r0/sync/sync_events.rs

@@ -14,7 +14,7 @@ use ruma_events::{
     to_device::AnyToDeviceEvent,
     EventJson,
 };
-use ruma_identifiers::RoomId;
+use ruma_identifiers::{RoomId, UserId};
 use serde::{Deserialize, Serialize};
 
 use crate::r0::{filter::FilterDefinition, keys::KeyAlgorithm};