Fix documentation of "must be vacuumed within" warning.

Warnings start 10M transactions before xidStopLimit, which is 11M
transactions before wraparound.  The sample WARNING output showed a
value greater than 11M, and its HINT message predated commit
25ec228e.  Hence, the sample was
impossible.  Back-patch to 9.5 (all supported versions).

doc/src/sgml/maintenance.sgml

@@ -606,14 +606,13 @@ SELECT datname, age(datfrozenxid) FROM pg_database;
    </para>
 
    <para>
-    If for some reason autovacuum fails to clear old XIDs from a table,
-    the system will begin to emit warning messages like this when the
-    database's oldest XIDs reach ten million transactions from the wraparound
-    point:
+    If for some reason autovacuum fails to clear old XIDs from a table, the
+    system will begin to emit warning messages like this when the database's
+    oldest XIDs reach eleven million transactions from the wraparound point:
 
 <programlisting>
-WARNING:  database "mydb" must be vacuumed within 177009986 transactions
-HINT:  To avoid a database shutdown, execute a database-wide VACUUM in "mydb".
+WARNING:  database "mydb" must be vacuumed within 10985967 transactions
+HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
 </programlisting>
 
     (A manual <command>VACUUM</command> should fix the problem, as suggested by the