pg_upgrade: document that --link should be used with --check

Backpatch to 9.2.

doc/src/sgml/pgupgrade.sgml

@@ -372,7 +372,9 @@ pg_upgrade.exe
      and then do the upgrade. You can use <command>pg_upgrade --check</>
      to perform only the checks, even if the old server is still
      running. <command>pg_upgrade --check</> will also outline any
-     manual adjustments you will need to make after the upgrade.
+     manual adjustments you will need to make after the upgrade.  If you
+     are going to be using link mode, you should use the <option>--link</>
+     option with <option>--check</option> to enable link-mode-specific checks.
      <command>pg_upgrade</> requires write permission in the current directory.
     </para>