pg_upgrade doc: link mode additions

Mention that link mode uses less disk space, and uses junction points on
Windows.

Backpatch to 9.3.

doc/src/sgml/pgupgrade.sgml

@@ -122,7 +122,8 @@
      <varlistentry>
       <term><option>-k</option></term>
       <term><option>--link</option></term>
-      <listitem><para>use hard links instead of copying files to the new cluster</para></listitem>
+      <listitem><para>use hard links instead of copying files to the new
+      cluster (use junction points on Windows)</para></listitem>
      </varlistentry>
 
      <varlistentry>
@@ -333,7 +334,8 @@ NET STOP pgsql-8.3  (<productname>PostgreSQL</> 8.3 and older used a different s
 
     <para>
      If you use link mode, the upgrade will be much faster (no file
-     copying), but you will not be able to access your old cluster
+     copying) and use less disk space, 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.  (Tablespaces and <filename>pg_xlog</> can be on