Clarify that we don't need tyo recheck

src/DeviceListener.js

@@ -110,6 +110,9 @@ export default class DeviceListener {
     _onWillUpdateDevices = async (users) => {
         const myUserId = MatrixClientPeg.get().getUserId();
         if (users.includes(myUserId)) this._ensureDeviceIdsAtStartPopulated();
+
+        // No need to do a recheck here: we just need to get a snapshot of our devices
+        // before we download asny new ones.
     }
 
     _onDevicesUpdated = (users) => {