Adjust VACUUM hastup LP_REDIRECT comments.

The term "truncation" has been ambiguous since commit 10a8d13823 added
line pointer array truncation during heap pruning.  Clear things up by
specifying that we're talking about rel truncation here, to match nearby
comments that apply to tuples with storage.

src/backend/access/heap/vacuumlazy.c

@@ -1595,7 +1595,8 @@ retry:
 		/* Redirect items mustn't be touched */
 		if (ItemIdIsRedirected(itemid))
 		{
-			prunestate->hastup = true;	/* page won't be truncatable */
+			/* page makes rel truncation unsafe */
+			prunestate->hastup = true;
 			continue;
 		}