Correct nbtree page split lock coupling comment.

There is no reason to distinguish between readers and writers here.

src/backend/access/nbtree/nbtinsert.c

@@ -1861,11 +1861,9 @@ _bt_split(Relation rel, BTScanInsert itup_key, Buffer buf, Buffer cbuf,
 	}
 
 	/*
-	 * We have to grab the right sibling (if any) and fix the prev pointer
-	 * there. We are guaranteed that this is deadlock-free since no other
-	 * writer will be holding a lock on that page and trying to move left, and
-	 * all readers release locks on a page before trying to fetch its
-	 * neighbors.
+	 * We have to grab the original right sibling (if any) and update its prev
+	 * link.  We are guaranteed that this is deadlock-free, since we couple
+	 * the locks in the standard order: left to right.
 	 */
 	if (!isrightmost)
 	{