Correct more obsolete nbtree page split comments.

Commit 3f342839 corrected obsolete comments about buffer locks at the
main _bt_insert_parent() call site, but missed similar obsolete comments
above _bt_insert_parent() itself.  Both sets of comments were rendered
obsolete by commit 40dae7ec53, which made the nbtree page split
algorithm more robust.  Fix the comments that were missed the first time
around now.

In passing, refine a related _bt_insert_parent() comment about
re-finding the parent page to insert new downlink.

src/backend/access/nbtree/nbtinsert.c

@@ -1687,13 +1687,15 @@ _bt_split(Relation rel, BTScanInsert itup_key, Buffer buf, Buffer cbuf,
 }
 
 /*
- * _bt_insert_parent() -- Insert downlink into parent after a page split.
+ * _bt_insert_parent() -- Insert downlink into parent, completing split.
  *
  * On entry, buf and rbuf are the left and right split pages, which we
- * still hold write locks on per the L&Y algorithm.  We release the
- * write locks once we have write lock on the parent page.  (Any sooner,
- * and it'd be possible for some other process to try to split or delete
- * one of these pages, and get confused because it cannot find the downlink.)
+ * still hold write locks on.  Both locks will be released here.  We
+ * release the rbuf lock once we have a write lock on the page that we
+ * intend to insert a downlink to rbuf on (i.e. buf's current parent page).
+ * The lock on buf is released at the same point as the lock on the parent
+ * page, since buf's INCOMPLETE_SPLIT flag must be cleared by the same
+ * atomic operation that completes the split by inserting a new downlink.
  *
  * stack - stack showing how we got here.  Will be NULL when splitting true
  *			root, or during concurrent root split, where we can be inefficient
@@ -1771,11 +1773,13 @@ _bt_insert_parent(Relation rel,
 		BTreeInnerTupleSetDownLink(new_item, rbknum);
 
 		/*
-		 * Find the parent buffer and get the parent page.
+		 * Re-find and write lock the parent of buf.
 		 *
-		 * Oops - if we were moved right then we need to change stack item! We
-		 * want to find parent pointing to where we are, right ?	- vadim
-		 * 05/27/97
+		 * It's possible that the location of buf's downlink has changed
+		 * since our initial _bt_search() descent.  _bt_getstackbuf() will
+		 * detect and recover from this, updating the stack, which ensures
+		 * that the new downlink will be inserted at the correct offset.
+		 * Even buf's parent may have changed.
 		 */
 		stack->bts_btentry = bknum;
 		pbuf = _bt_getstackbuf(rel, stack);