Update btree_xlog_delete() comments.

Commit fe97c61 updated LP_DEAD item deletion comments, but missed a
minor discrepancy on the REDO side.  Fix it now.

In passing, don't talk about the btree_xlog_vacuum() behavior within
btree_xlog_delete().  The reliance on XLOG_HEAP2_CLEANUP_INFO records
for recovery conflicts is already discussed within btvacuumpage() and
mentioned again in passing above btree_xlog_vacuum(), which seems
sufficient.

Author: petergeoghegan

src/backend/access/nbtree/nbtree.c

@@ -1208,9 +1208,10 @@ btvacuumpage(BTVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
 				 * as long as the callback function only considers whether the
 				 * index tuple refers to pre-cutoff heap tuples that were
 				 * certainly already pruned away during VACUUM's initial heap
-				 * scan by the time we get here. (We can rely on conflicts
-				 * produced by heap pruning, rather than producing our own
-				 * now.)
+				 * scan by the time we get here. (XLOG_HEAP2_CLEANUP_INFO
+				 * records produce conflicts using a latestRemovedXid value
+				 * for the entire VACUUM, so there is no need to produce our
+				 * own conflict now.)
 				 *
 				 * Backends with snapshots acquired after a VACUUM starts but
 				 * before it finishes could have a RecentGlobalXmin with a

src/backend/access/nbtree/nbtxlog.c

@@ -428,14 +428,7 @@ btree_xlog_delete(XLogReaderState *record)
 
 	/*
 	 * If we have any conflict processing to do, it must happen before we
-	 * update the page.
-	 *
-	 * Btree delete records can conflict with standby queries.  You might
-	 * think that vacuum records would conflict as well, but we've handled
-	 * that already.  XLOG_HEAP2_CLEANUP_INFO records provide the highest xid
-	 * cleaned by the vacuum of the heap and so we can resolve any conflicts
-	 * just once when that arrives.  After that we know that no conflicts
-	 * exist from individual btree vacuum records on that index.
+	 * update the page
 	 */
 	if (InHotStandby)
 	{
@@ -463,10 +456,7 @@ btree_xlog_delete(XLogReaderState *record)
 			PageIndexMultiDelete(page, unused, xlrec->nitems);
 		}
 
-		/*
-		 * Mark the page as not containing any LP_DEAD items --- see comments
-		 * in _bt_delitems_delete().
-		 */
+		/* Mark the page as not containing any LP_DEAD items */
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		opaque->btpo_flags &= ~BTP_HAS_GARBAGE;