Update comment atop of ReorderBufferQueueMessage().

The comments atop of this function describes behaviour in case of a
transactional WAL message only, but it accepts both transactional and
non-transactional WAL messages. Update the comments to describe
behaviour in case of non-transactional WAL message as well.

Ashutosh Bapat, rephrased by Amit Kapila
Discussion: https://postgr.es/m/CAGEoWWTTzNzHOi8bj0wfAo1siGi-YEh6wqH1oaz4DrkTJ6HbTQ@mail.gmail.com

src/backend/replication/logical/reorderbuffer.c

@@ -783,7 +783,8 @@ ReorderBufferQueueChange(ReorderBuffer *rb, TransactionId xid, XLogRecPtr lsn,
 }
 
 /*
- * Queue message into a transaction so it can be processed upon commit.
+ * A transactional message is queued to be processed upon commit and a
+ * non-transactional message gets processed immediately.
  */
 void
 ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,