Fix ancient docs/comments thinko: XID comparison is mod 2^32, not 2^31.

Pointed out by Gianni Ciolli.

doc/src/sgml/maintenance.sgml

@@ -401,7 +401,7 @@
     as <literal>FrozenXID</>.  This XID does not follow the normal XID
     comparison rules and is always considered older
     than every normal XID. Normal XIDs are
-    compared using modulo-2<superscript>31</> arithmetic. This means
+    compared using modulo-2<superscript>32</> arithmetic. This means
     that for every normal XID, there are two billion XIDs that are
     <quote>older</> and two billion that are <quote>newer</>; another
     way to say it is that the normal XID space is circular with no

src/backend/access/transam/transam.c

@@ -304,7 +304,7 @@ TransactionIdPrecedes(TransactionId id1, TransactionId id2)
 {
 	/*
 	 * If either ID is a permanent XID then we can just do unsigned
-	 * comparison.	If both are normal, do a modulo-2^31 comparison.
+	 * comparison.	If both are normal, do a modulo-2^32 comparison.
 	 */
 	int32		diff;