Update btree_xlog_delete() comments.

Commit fe97c61c 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.

src/backend/access/nbtree/nbtree.c

@@ -1208,9 +1208,10 @@ restart:
 				 * 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