store [nfc]: Add comment to verify RealmUserUpdateEvent handling is exhaustive

This was prompted by noticing recently that we weren't updating
[User.isActive]:
  zulip#816 (comment)

After fixing that, I looked and found there were actually three
other fields on User which we weren't updating -- a latent bug,
since we never looked at those fields, but a bug.  Fixed that in
the preceding commit.

Now add a comment showing my work on verifying that that's it,
and giving us a head start on re-verifying that in the future
with whatever's changed in the API between now and then.

Author: gnprice

lib/model/store.dart

@@ -465,6 +465,10 @@ class PerAccountStore extends ChangeNotifier with ChannelStore, MessageStore {
         if (user == null) {
           return; // TODO log
         }
+        // Fields on [User] not updated here:
+        //   userId, dateJoined, isBot, botType, isSystemBot
+        // Each of those is immutable on any given user, and there are no
+        // RealmUserUpdateEvent events that update them.
         if (event.fullName != null)       user.fullName       = event.fullName!;
         if (event.avatarUrl != null)      user.avatarUrl      = event.avatarUrl!;
         if (event.avatarVersion != null)  user.avatarVersion  = event.avatarVersion!;