Ref links for 'Preferred language for notices'

Someone on IRC was confused about this, and we noticed the field was missing from the patron field descriptions.

I added links to help readers get around to relevant sections... hopefully it's a bit clearer.

We also discussed which notices were affected by preferred language and which were not (slips, most probably). I wanted to check, but I still can't make translations work in my local installations (local and ktd), so it will have to wait till later.