Transifex is our system for people contributing translations for localisation. You don’t need to worry about that and can pretend it doesn’t exist.
But this is a good question - most of the below are (I think) self-evident except for new_user_tips. In any case it might be a good idea to have an ⓘ explanation nearby.
Available interpolation keys: site_name, new_user_tips, base_url, username, name, name_or_username
You’d have to go digging into the code to find system_messages.usage_tips.text_body_template.
This is something that could definitely be improved so it’s apparent in the editor.