Add more comments to explain why replication log queue could never be empty for normal replication queue

In HBASE-28037, Xiaolin Ha found that there could be a very small window that even for a normal replication source, its queue could be empty.

This is because we will only enqueue the wal file to the queue in postLogRoll, where the old WAL writer has already been closed, so if the replication is fast enough, we could reach the end of the queue before enqueuing the new wal file.

The code for branch-2+ has been refactored a lot so we opened a new issue for fixing this.