Bug 34438: OPAC self registration form does not include lang (preferred language for notices) field

I wasn't sure this valued a mention in the manual... I figured if someone was looking for it in an older version, they might see it was only added in 23.11. Let me know what you think.

I took the opportunity to update the screenshot with all the newish fields like the pronouns and gender options, etc. and to review the pref.