Add comment about keeping PD_ALL_VISIBLE and VM in sync

The comment above heap_xlog_visible() about the critical integrity
requirement for PD_ALL_VISIBLE and the visibility map should also be in
heap_xlog_prune_freeze() where we set PD_ALL_VISIBLE.

Oversight in add323da40

Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_ZMw6Npd_qm2KM%2BFwQ3cMOMx1Dh3VMhp8-V7SOLxdK9-g%40mail.gmail.com

src/backend/access/heap/heapam_xlog.c

@@ -157,6 +157,12 @@ heap_xlog_prune_freeze(XLogReaderState *record)
 		/* There should be no more data */
 		Assert((char *) frz_offsets == dataptr + datalen);
 
+		/*
+		 * The critical integrity requirement here is that we must never end
+		 * up with the visibility map bit set and the page-level
+		 * PD_ALL_VISIBLE bit unset.  If that were to occur, a subsequent page
+		 * modification would fail to clear the visibility map bit.
+		 */
 		if (vmflags & VISIBILITYMAP_VALID_BITS)
 			PageSetAllVisible(page);