pg_upgrade: document link options

Document that tablespaces and pg_xlog can be on different file systems
for pg_upgrade --link mode.
Backpatch to 9.3.

doc/src/sgml/pgupgrade.sgml

@@ -335,7 +335,8 @@ NET STOP pgsql-8.3  (<productname>PostgreSQL</> 8.3 and older used a different s
      copying), but you will not be able to access your old cluster
      once you start the new cluster after the upgrade.  Link mode also
      requires that the old and new cluster data directories be in the
-     same file system.  See <literal>pg_upgrade --help</> for a full
+     same file system.  (Tablespaces and <filename>pg_xlog</> can be on
+     different file systems.)  See <literal>pg_upgrade --help</> for a full
      list of options.
     </para>