add some comments while debugging a problem

src/matrix/Session.js

@@ -287,7 +287,7 @@ export class Session {
     }
 
     /**
-     * @internal called when coming back online
+     * @internal called from session container when coming back online and catchup syncs have finished.
      * @param  {Object} lastVersionResponse a response from /versions, which is polled while offline,
      *                                      and useful to store so we can later tell what capabilities
      *                                      our homeserver has.

src/matrix/e2ee/DeviceTracker.js

@@ -45,6 +45,13 @@ export class DeviceTracker {
 
     async writeDeviceChanges(deviceLists, txn) {
         const {userIdentities} = txn;
+        // TODO: should we also look at left here to handle this?:
+        // the usual problem here is that you share a room with a user,
+        // go offline, the remote user leaves the room, changes their devices,
+        // then rejoins the room you share (or another room).
+        // At which point you come online, all of this happens in the gap, 
+        // and you don't notice that they ever left, 
+        // and so the client doesn't invalidate their device cache for the user
         if (Array.isArray(deviceLists.changed) && deviceLists.changed.length) {
             await Promise.all(deviceLists.changed.map(async userId => {
                 const user = await userIdentities.get(userId);

src/matrix/storage/idb/Transaction.js

@@ -121,6 +121,7 @@ export class Transaction {
     }
 
     abort() {
+        // TODO: should we wrap the exception in a StorageError?
         this._txn.abort();
     }
 }