[Tails-l10n] String clarification on tails-iuk.pot

Hi all,

Translators on Transifex are requesting clarification of the following
string:

#: ../lib/Tails/IUK/Frontend.pm:279
msgid "XXX HEJ XXX"
msgstr ""

This appears in https://git-tails.immerda.ch/iuk/plain/po/tails-iuk.pot.

It would be a good idea to add a comment to the string explaining how
translators should deal with it.

Thanks!