Add tests and docs for Realm.WriteCopy with Sync

[sync-by-unito]

Core changed the behavior of WriteCopy for synchronized Realms to remove the client file identifier. This means that we can now WriteCopy a Realm owned by user A and open it with user B and not have to redownload the data/experience a client reset. We should get that for free since we already use WriteCopy, but we should:

1. Write tests that verify that this is indeed the behavior, including error cases where the user tries to copy a Realm that hasn't finished uploading all its changes.
2. Update the API docs to clarify what the behavior for sync configurations is.