Fix LDAP Extended Query example.

The old one was not valid LDAP filter syntax.
Improve Authentication Containers examples.

* The first example was wrong (cf. ldap_backed() in auth.inc which
  looks for a DC= part to determine a full DN.
* The second example now show the realtive DNs.
Don't gettext() the example strings.

They must not be translated.
Merge pull request #659 from mss/extended-query-example-work

Improve LDAP DN examples