Add C comment about the fact that the autovacuum limit can go backwards

by 3, but that is it OK.

src/backend/postmaster/autovacuum.c

@@ -1108,6 +1108,7 @@ do_start_worker(void)
 	recentXid = ReadNewTransactionId();
 	xidForceLimit = recentXid - autovacuum_freeze_max_age;
 	/* ensure it's a "normal" XID, else TransactionIdPrecedes misbehaves */
+	/* this can cause the limit to go backwards by 3, but that's OK */
 	if (xidForceLimit < FirstNormalTransactionId)
 		xidForceLimit -= FirstNormalTransactionId;