doc: add translation disclaimer

Add the translation disclaimer in English as reference
for other languages.  Translations must include this disclaimer
in their language so that readers are properly informed.

This very same patch updates the Italian translation accordingly.

Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Acked-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>