Update obsolete heap pruning comments.

Add new comments that spell out what VACUUM expects from heap pruning:
pruning must never leave behind DEAD tuples that still have tuple
storage.  This has at least been the case since commit 8523492d, which
established the principle that vacuumlazy.c doesn't have to deal with
DEAD tuples that still have tuple storage directly, except perhaps by
simply retrying pruning (to handle a rare corner case involving
concurrent transaction abort).

In passing, update some references to old symbol names that were missed
by the snapshot scalability work (specifically commit dc7420c2c9).

src/backend/access/heap/pruneheap.c

@@ -488,17 +488,25 @@ heap_prune_satisfies_vacuum(PruneState *prstate, HeapTuple tup, Buffer buffer)
  * the HOT chain is pruned by removing all DEAD tuples at the start of the HOT
  * chain.  We also prune any RECENTLY_DEAD tuples preceding a DEAD tuple.
  * This is OK because a RECENTLY_DEAD tuple preceding a DEAD tuple is really
- * DEAD, the OldestXmin test is just too coarse to detect it.
+ * DEAD, the heap_prune_satisfies_vacuum test is just too coarse to detect it.
+ *
+ * In general, pruning must never leave behind a DEAD tuple that still has
+ * tuple storage.  VACUUM isn't prepared to deal with that case.  That's why
+ * VACUUM prunes the same heap page a second time (without dropping its lock
+ * in the interim) when it sees a newly DEAD tuple that we initially saw as
+ * in-progress.  Retrying pruning like this can only happen when an inserting
+ * transaction concurrently aborts.
  *
  * The root line pointer is redirected to the tuple immediately after the
  * latest DEAD tuple.  If all tuples in the chain are DEAD, the root line
  * pointer is marked LP_DEAD.  (This includes the case of a DEAD simple
  * tuple, which we treat as a chain of length 1.)
  *
- * OldestXmin is the cutoff XID used to identify dead tuples.
+ * prstate->vistest is used to distinguish whether tuples are DEAD or
+ * RECENTLY_DEAD.
  *
  * We don't actually change the page here, except perhaps for hint-bit updates
- * caused by HeapTupleSatisfiesVacuum.  We just add entries to the arrays in
+ * caused by heap_prune_satisfies_vacuum.  We just add entries to the arrays in
  * prstate showing the changes to be made.  Items to be redirected are added
  * to the redirected[] array (two entries per redirection); items to be set to
  * LP_DEAD state are added to nowdead[]; and items to be set to LP_UNUSED
@@ -694,9 +702,9 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate)
 		/*
 		 * Remember the last DEAD tuple seen.  We will advance past
 		 * RECENTLY_DEAD tuples just in case there's a DEAD one after them;
-		 * but we can't advance past anything else.  (XXX is it really worth
-		 * continuing to scan beyond RECENTLY_DEAD?  The case where we will
-		 * find another DEAD tuple is a fairly unusual corner case.)
+		 * but we can't advance past anything else.  We have to make sure that
+		 * we don't miss any DEAD tuples, since DEAD tuples that still have
+		 * tuple storage after pruning will confuse VACUUM.
 		 */
 		if (tupdead)
 		{