Locale documentation tweaks

Fix a confusing piece of documentation about "only serving the newest" item when there's a duplicate by `content_id` and `locale`: this de-duping is only actually done when expanding links.

Also, give some details of the locale settings we use, with links to standards.

I believe I've linked to the correct standards, but it would be good to have the assertions I've made here double-checked.
Consistency: you use “2 letter” here and “two-letter” a couple of lines down.